GCSE Computer Science Key Terms & Vocabulary

Anticipating misuse

a programmer thinks about how a user could accidentally or deliberately cause a program to fail, and writes code to prevent harm.

One-dimensional array

a single, ordered list of values held under a single variable name.

Two-dimensional array

a grid of values organised by rows and columns.

2D array

useful for representing data in a table or grid form — spreadsheets, game boards, etc.

Array items

accessed using their index, e.g. a[3] is the fourth item in a zero-indexed array.

Authentication

the process of checking who a user is.

Boundary values

tested because edge cases are where off-by-one and equality-vs-strict-inequality bugs often hide.

Boundary test data

data at the edges of the valid range — typically the minimum, maximum, and the values just outside.

Characters

converted to numeric codes so the computer can store them as binary patterns in memory.

Chessboard

best modelled by an 8 × 8 2D array (one row and one column for each square).

Comment

a note added to code to explain its purpose; it is ignored by the computer when the program runs.

Comments

not executed — they are there only for the human reader, not the computer.

Constant

a named value that is fixed at declaration and cannot change while the program runs.

Defensive design

writing programs that anticipate unexpected or incorrect use and handle it gracefully instead of crashing.

Erroneous data

data the program should reject as invalid — the test is whether the rejection actually happens.

Erroneous test data

data the program should reject — wrong type, out of range, or invalid format.

Error handling

code that catches runtime errors (e.g. dividing by zero, opening a missing file) and responds sensibly instead of crashing.

Indentation

the consistent use of horizontal spacing to show that statements belong inside a block (e.g. a loop or IF statement).

Indentation

essential to making nested control structures readable, especially in languages without explicit end markers.

Input sanitisation

the process of removing or replacing dangerous or unexpected characters from user input before the program uses it.

Integer

a whole number with no decimal part (e.g. 42, -3, 0).

Iteration

repeating a block of code, with each repetition called a pass through the loop.

Local variable

a variable that exists only inside the subroutine that declared it.

Logic error

the code runs but produces the wrong result — the syntax is correct, the reasoning is not.

Logic errors

usually the hardest to find because the program still runs — only the output is wrong.

Maintainability

how easily a program can be read, understood, changed, and debugged, especially by someone other than the original programmer.

Modular programs

typically easier to debug because each module's behaviour can be examined in isolation.

Modular code

easier to test than one large block, because each module has clear boundaries.

Modularity

splitting a program into independent, well-defined parts (modules or subroutines).

Nested loop

a loop placed inside another loop; the inner loop completes all its iterations for every one iteration of the outer loop.

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Access control

limiting which users can access which resources.

Anti-virus software alone

not enough — social engineering bypasses technical controls by attacking the human.

Fingerprint scan

a biometric authentication factor (PINs, USB keys, SMS codes are knowledge/possession factors, not biometrics).

Black-box testing

the tester has no inside knowledge of the system — they probe it as an outsider would.

Blagging (also called pretexting)

inventing a believable story to convince a victim to grant access or hand over information.

Blagging

a story-based manipulation — it does not involve writing malicious code or hacking a server.

CAPTCHA's main job

to tell humans apart from automated bots on web forms.

CAPTCHAs

not a replacement for passwords on a login form — they supplement rather than replace authentication.

Email confirmation alone

NOT a complete two-factor authentication system — it's a single account-creation check, not an ongoing second login factor.

Cyber security

the protection of computers, devices, networks and data from unauthorised access, damage and disruption.

Cyber security

everyone's responsibility, not just the IT department's — most successful attacks involve human error (e.g. clicking a phishing link).

Primary purpose

to prevent unauthorised access to data and systems, and to keep services running reliably.

Default passwords

risky because attackers already know them — they're published in manuals and on enumeration sites.

Default credentials

widely published in vendor manuals and online lists; that's the fundamental reason to change them.

Forcing frequent password changes

now considered bad practice — users respond by creating weaker, predictable variants. NIST and NCSC both discourage scheduled rotation.

Malware

software intentionally designed to harm a computer, steal data, or disrupt services.

Malware

malicious software designed to harm systems — it is not legitimate productivity software.

Malware

created with malicious intent — this is the defining property, not how it's delivered.

Malware

short for "malicious software" — software written with harmful intent.

Malware

short for malicious software.

Cyber security

not concerned with CPU clock speed, screen brightness, file compression, or language translation — those are unrelated computing concerns.

Outdated software

NOT more secure than updated software — it's almost always less secure because known vulnerabilities remain unfixed.

Outdated software

a risk because its known vulnerabilities remain unpatched and attackers can exploit them at will.

Good password rule

to use a long, unique passphrase for each site (NCSC's "three random words" guidance is a UK example).

Common policy rule

a minimum length of 8–12 characters (NIST now recommends at least 15).

Password policy

a set of rules governing how passwords are created and used on a system.

Patch

a security fix for known bugs in software.

Penetration testing

an authorised simulated cyberattack designed to find weaknesses in a system before real attackers do.

Convincing pharmed page

designed to look identical to the legitimate site so users don't notice the substitution.

One defence

to check the exact site address and HTTPS padlock before entering credentials — a pharmed page often has an incorrect URL or missing/invalid TLS certificate.

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ASCII

limited in modern use because it cannot encode many non-English characters (no Chinese, Arabic, Cyrillic, accented Latin).

Binary

the base computers use natively.

Bit

the smallest unit of digital information — a single binary digit holding 0 or 1.

There

8 bits in a byte.

There

8 bits in 1 byte.

Byte

made up of eight bits.

Colour depth

the number of bits used to represent the colour of a single pixel.

Colour depth

bits per pixel, not to the height of the image in pixels (that's part of resolution).

Compression

useful because compressed files take less storage space and less bandwidth (transmit faster).

Compression

not always lossy by default — many widely-used compressed formats (ZIP, PNG, FLAC) are lossless.

Decimal

base 10 — it uses ten distinct digits (0-9).

Gigabyte

larger than a megabyte (10⁹ bytes vs 10⁶ bytes).

Hexadecimal

a more compact way of writing binary values.

Hex

used because each digit equals 4 bits — making conversion to binary trivial.

Hex

used for memory addresses because it is compact and converts cleanly to binary, the form the hardware uses.

Hex

base 16, not base 8 (base 8 is octal — a different system using digits 0-7).

Hex

not used because it is faster for the CPU — the CPU still works in binary internally. Hex exists for human convenience.

Hex

shorter than binary because four binary digits collapse into a single hex digit, making it easier for programmers to read and write.

Hexadecimal symbols

0-9 followed by A, B, C, D, E, F (representing decimal values 10-15).

Character's Huffman code

read off by following the path from the root to that character's leaf, recording 0 for each left branch and 1 for each right branch.

Huffman coding

a lossless compression method — no data is lost.

Huffman coding

not a lossy method.

Huffman codes

prefix-free — no code is the prefix of another — so a decoder can unambiguously work out where one code ends and the next begins.

Huffman codes

typically shorter for more frequent characters.

Huffman coding

most effective when character frequencies are uneven — the more skewed the distribution, the bigger the savings.

If the sample rate

too low, the digital recording loses detail and high-frequency content from the original sound.

Nibble

4 bits — half a byte (the size of one hexadecimal digit).

Overflow

when the result of an addition needs more bits than the register can hold — the high bit is lost or flagged.

Overflow flag

a CPU status bit that indicates the most recent arithmetic operation exceeded the register's capacity.

Pixel

the smallest individually addressable dot in a raster image.

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Accumulator (ACC)

the register that holds the data the ALU is currently working on and the result of the most recent ALU operation.

Assembly language

a low-level language — its instructions are mnemonics for the instruction set of one specific CPU architecture.

Assembly language

mostly made of short mnemonics (like MOV, ADD, SUB, LDR) that represent individual CPU instructions.

Web browser

application software — it performs a task for the user.

CPU cache

very fast memory placed close to (or on) the CPU that holds recently used instructions and data.

CD-ROM

optical (read by a laser), not magnetic storage.

Higher clock speed

more clock cycles per second, so more instructions can be executed per second (all else equal).

Compiled programs

usually distributed as binary executables; interpreted programs are typically distributed as source code that the user's interpreter then runs.

More CPU cores

more separate processing units, so multiple tasks (or threads) can run in parallel.

CPU

the component that fetches instructions from memory in a Von Neumann machine.

CPU

a hardware component (a physical silicon chip), not software.

Cycle

commonly written as fetch – decode – execute in that order.

Disk defragmenter

utility software that rearranges fragmented files on a hard drive so each file's data is stored in contiguous sectors.

Assembly

easier to read and write than raw machine code — mnemonics are easier for humans than long binary numbers.

Firmware

software stored permanently inside a hardware component (typically on ROM or flash memory) that provides low-level control of that device — for example the BIOS or the controller in a printer.

Harvard architecture

the main alternative — it uses physically separate memories and buses for code and data, removing the bottleneck.

HDDs

the most fragile of the three when running because their spinning platters and moving heads are easily damaged by shock.

High-level languages

the level that is closer to human language than to the CPU.

High-level languages

preferred for learning programming because they are easier to read, write, and debug.

Hardware

the physical/electronic parts of a computer; software is the set of instructions (programs and data) that tell the hardware what to do.

Machine code

binary instructions that the CPU can execute directly without any further translation.

Machine code

a low-level language, not high-level.

Multitasking

the OS's ability to run multiple programs apparently at once, by switching the CPU between them rapidly.

NAND gate

an AND gate followed by a NOT — the output is the inversion of AND.

Operating system

software, not hardware, even though it's stored on a hardware device (a disk).

Operating system

classed as system software, not application software.

Python

a high-level language — it uses readable English-like keywords (if, while, for, def).

Python

most commonly used as an interpreted, high-level language at school level.

At school level Python

the canonical example of an interpreted language.

RAM

volatile memory — its contents disappear when the computer is switched off.

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Authentication

needed on a network to restrict access to authorised users only.

Biometric authentication

harder to fake than a password because the verifying feature is part of the user's body.

If encrypted data

intercepted, the attacker only sees scrambled ciphertext, not the original plaintext.

Common protocol

needed between two computers so they both agree on how data is sent and received.

Key disadvantage

the greater risk of malware spreading widely from one infected device to others on the network.

Encryption

used on a network so that intercepted data cannot be read by attackers.

Firewall

a filter for network traffic that allows or blocks packets based on configured rules.

Firewall

not a complete defence — other security layers (antivirus, patching, user training) are still needed.

Firewall

usually placed between a network and the internet — at the boundary between trusted and untrusted networks.

HTTP (Hypertext Transfer Protocol)

a typical network protocol — used for transferring web pages.

HTTP

used for transferring web pages and other resources between a web server and a web browser.

HTTPS

now the recommended default for all websites, not just login pages — modern browsers warn users when a site is plain HTTP.

IMAP

the protocol that leaves email on the server until the user deletes it (contrast with POP3, which downloads and removes).

IMAP

for retrieving email — it is not used for transferring web pages between servers (that's HTTP).

Internet

the world's largest WAN — spanning the entire globe.

IP address

a unique number identifying a device on a network.

IP's main job

addressing each packet (source and destination IP addresses) and routing it across the network.

Even strongly encrypted data

at risk if the encryption key itself is leaked.

Model

split into layers so each layer handles one specific job — protocols can change at one layer without affecting the others.

MAC address

a unique identifier assigned to a network adapter, used at the Link layer.

MAC address filtering

not impossible to bypass — spoofing is a well-known attack.

MAC address filtering

usually configured on the network router or wireless access point.

Computer network

best defined as two or more devices that share data via a communication link.

Computer network

two or more connected computers that can share data.

PAN (Personal Area Network)

the smallest network type, typically spanning a few metres around a single user.

Network protocol

a set of agreed rules for communication between devices.

Network protocol

a set of rules for communication between devices.

Server

a single device that can manage network access — handling authentication, file storage, and shared services.

SMTP

mainly used for sending email between mail servers.

SMTP

used to send email between mail servers (and from a mail client to its outgoing server).

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Abstraction

removing or hiding detail that isn't needed to solve the problem at hand, focusing only on the essential information.

Abstraction

ignoring details that are not relevant to solving the problem.

Algorithm

a precise, finite sequence of steps for solving a problem.

Algorithm

a sequence (an ordered set) of steps to complete a task — "a set of instructions" is too vague because order matters. An algorithm is not the same as a program; a program is one implementation of an algorithm.

AQA pseudo-code

language-independent — the exam does not enforce a single strict syntax, only the broad conventions in the AQA reference language document.

Binary search

faster than linear search on large sorted data sets — the log₂ n vs n gap widens rapidly.

If the target

less than the middle item, binary search continues with the lower half of the list (and discards the upper half).

Binary search

not always faster — on very small lists, or when the target is the first element, linear can win.

Bubble sort

in-place: it uses only a small fixed amount of extra memory (O(1) auxiliary).

Bubble sort

the more space-efficient algorithm on large data — it works in place using O(1) auxiliary memory.

Bubble sort's worst case

about n(n-1)/2 comparisons — quadratic time.

Decomposition

useful because smaller sub-problems are easier to understand, design, code, test, and reuse than the whole problem at once.

Decomposition

breaking a complex problem down into smaller, more manageable sub-problems.

Decomposition

splitting a problem into sub-problems — it does not combine many small problems into one larger problem.

If the target

the very first item in the list, linear search needs just one comparison.

Linear search

the search that works on unsorted data; binary search requires sorted data.

Linear search

sometimes preferred even on sorted data because it is simpler to write, read, and trace.

Merge sort

called divide and conquer because it splits the original problem into smaller sub-problems (sub-lists), solves them, and combines the results.

Merge sort

the one that uses the divide-and-conquer strategy.

Merge sort

faster than bubble sort on large data sets (O(n log n) vs O(n²)).

Merge sort

generally faster than bubble sort on large data sets (O(n log n) vs O(n²)).

Merge sort

preferred for sorting very large data sets where speed matters most.

Pseudo-code

used for describing algorithms in plain, language-independent steps before they're translated into a specific programming language.

Trace table

more useful when it records every change of each variable, not just the final values.

Trace tables

most useful for finding logic errors (the program runs but produces the wrong output).

Trace table

a row-by-row record of the values held by each variable at each step of an algorithm.

London Tube map

the classic example of abstraction — lines are drawn straight to show connectivity, not the geographically exact route.

Algorithm efficiency

typically compared using two resources: time taken (running speed) and memory used.

Flowchart arrows show the order of execution between the shapes.

The author of the algorithm is not a factor that affects its efficiency comparison — only its steps/memory/scaling do.

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Data inconsistency

when the same fact is recorded with different values in different places — usually caused by missing updates to redundant copies.

Data redundancy

when the same data is stored in more than one place.

Database

an organised collection of data.

Typical real-world example

a school storing all pupil records (and class enrolment, attendance, grades) in a database.

Database

NOT just a folder of unrelated text files — the data is structured and the relationships are explicit.

Database

a structured store of related data with built-in query support — not a random text file or photo folder.

Database

designed for storing and retrieving large amounts of structured data.

Field

a single column of a database table — one attribute (name, age, year).

Field

NOT the entire row of data — it's a single column.

Foreign key

a field in one table that holds the primary key value of a record in another table — used to link the two.

Foreign key

used to link records across two tables (Students.class_id → Classes.id).

INSERT INTO

NOT used to delete records — that's the DELETE FROM statement.

Operation

called a "join" because two tables are joined into one combined result set.

Normalisation

the formal process that reduces redundancy in relational design by splitting into multiple tables according to normal-form rules.

Primary key

a unique identifier for each record in a table.

Record

a single row in a database table.

Record

one row representing a single entity in a database.

Record

a single row of data in a database table — typically describing one entity (one student, one book).

Redundancy

a problem because it wastes storage and risks inconsistency: an update missed in one place leaves contradictory data.

Relational database

a database that stores data in linked tables.

Relational database

the type of database that stores data in linked tables.

Relational design

chosen over a flat-file design because it reduces redundancy and the inconsistency that redundancy causes.

Table

a structured grid of rows and columns.

In AQA-style GCSE SQL, two tables are joined using a WHERE clause that matches the key fields: ... FROM A, B WHERE A.id = B.a_id.

At GCSE level, AQA's SQL questions are limited to two-table queries — they do not require students to write three- or four-table joins.

ASC stands for ascending order — smallest to largest, or A→Z for text.

SELECT title, name FROM Books, Authors WHERE Books.author_id = Authors.id returns each book's title plus its author's name.

A two-table query without a WHERE link condition produces every combination of rows (a Cartesian product).

A composite primary key uses two or more columns together for uniqueness — useful when no single column is unique on its own.

The best way to avoid storing a customer's address twice is to store the address once in a Customers table and link to it from Orders via a foreign key.

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One ethical/legal issue

liability when a self-driving car crashes — whose fault is it?

Cyber-security concern

that a hacked autonomous vehicle could be driven remotely — high consequence, low frequency.

Fully self-driving car

described as autonomous (SAE level 5 is the fully autonomous level).

Potential benefit

reducing road accidents caused by human error (impaired driving, distraction, fatigue).

Major ethical issue

how an autonomous car should decide in a no-win crash scenario — the so-called "trolley problem" applied to AVs.

Autonomous vehicle

a car (or other vehicle) capable of operating with reduced or no human input.

Black-hat hacker

an attacker working with malicious intent (theft, sabotage, extortion).

Disadvantage

that an internet connection is required to access stored files.

Privacy concern

that a third party can potentially see or lose your data — and may be subject to different jurisdictions / lawful-access regimes.

Cloud storage

data stored on remote servers operated by a third party, accessed over the internet.

UK 1990 anti-hacking law

the Computer Misuse Act.

Computer Misuse Act 1990

the UK law that makes unauthorised access to a computer illegal.

E-waste

discarded electronic devices and components.

E-waste

discarded electronic devices and components — distinct from paper waste, plastic bags or garden waste.

Encryption

important on public Wi-Fi because it stops eavesdroppers reading data in transit (HTTPS, VPN).

Ethical concern

long- term consent and the practical ability to remove the implant later — surgery to remove can carry significant risk.

Privacy issue

that constant body-monitoring data may be sent to a remote server (the vendor's, or a clinician's) — raising who-sees-what questions.

Computer-based implant

a computing device placed inside the body.

Documented security concern

that wirelessly-accessible implants could be hacked or interfered with — the US FDA has issued real-world pacemaker firmware advisories.

Benefit

that implants can monitor a condition continuously and treat it automatically (e.g. ICDs delivering a shock during arrhythmia).

Negative impact

increased screen time and the risk of compulsive use ("smartphone addiction").

Pacemaker

the implant that regulates a slow heartbeat.

There

a trade-off: more surveillance reduces personal privacy; more privacy can reduce the ability of authorities to investigate crime.

There

genuine debate about how to balance privacy rights with state security.

There

genuine tension between user privacy and state surveillance.

Common wearable

a smartwatch tracking heart rate and activity.

Governments argue surveillance

needed to detect terrorism, organised crime and child abuse.

Wearable technology

an electronic device worn on the body.

Privacy concern

the continuous tracking of health, activity and location data — Strava's heat-map incident revealed military base locations.

Positive impact

that wearables can encourage people to exercise more by setting step goals and providing feedback.

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