GCSE Computer Science glossary

A

Abstraction

Removing unnecessary detail so that only what matters for a problem is kept, such as a map that shows roads but not every tree. See also: Decomposition.

Algorithm

A finite, ordered set of steps that solves a problem or completes a task. See also: Pseudocode, Trace table.

ALU (arithmetic logic unit)

A part of the CPU that carries out arithmetic and logical operations, such as adding two numbers or comparing them. See also: CPU, Register.

ASCII

A character set that gives each basic English character a binary code, using 7 bits for each character in its standard form. See also: Character set, Unicode.

Authentication

Checking that a user is who they claim to be, for example with a password. See also: Brute force attack.

B

Binary

A number system that uses only two digits, 0 and 1, with place values that double each time: 1, 2, 4, 8 and so on. See also: Bit, Denary, Hexadecimal.

Binary search

A search for an item in a sorted list that checks the middle item and discards the half that cannot contain the target, repeating until the item is found or none remain. See also: Linear search.

Bit

A single binary digit, either 0 or 1; the smallest unit of data. See also: Byte, Binary.

Boolean

A data type with only two possible values, True or False. See also: Data type.

Brute force attack

Trying every possible password or key, or a very large number of guesses, until one works. See also: Authentication, Encryption.

Bubble sort

A sorting algorithm that repeatedly compares neighbouring items and swaps them if they are in the wrong order, until a full pass makes no swaps. See also: Algorithm.

Byte

A group of 8 bits. See also: Bit.

C

Cache

A small, very fast memory inside or close to the CPU that holds recently used data and instructions. See also: CPU, RAM.

Casting

Converting a value from one data type to another, such as turning the string "5" into the integer 5. See also: Data type, String.

Character set

A defined list of characters and the binary code used to represent each one. See also: ASCII, Unicode.

Client-server network

A network in which client devices request services or files and a central server provides them. See also: Protocol.

Constant

A named value that is set once and does not change while the program runs. See also: Variable.

CPU (central processing unit)

A component that fetches, decodes and executes instructions. See also: ALU, Fetch-decode-execute cycle.

D

Data type

A kind of value a piece of data holds, such as integer, real, Boolean, character or string. See also: Casting, Boolean.

Decomposition

Breaking a large problem into smaller parts that are easier to solve. See also: Abstraction.

Denary

Ordinary base-10 number system, using digits 0 to 9. See also: Binary, Hexadecimal.

E

Encryption

Scrambling data with a key so that only someone with the right key can read it. See also: Brute force attack.

F

Fetch-decode-execute cycle

A repeating process in which the CPU fetches an instruction from memory, decodes it, and carries it out. See also: CPU, Register.

Firewall

Software or hardware that monitors network traffic and blocks anything that breaks its rules. See also: Malware, Packet.

Function

A subroutine that returns a value to the code that called it. See also: Subroutine.

H

Hexadecimal

A base-16 number system using digits 0 to 9 and letters A to F, often used as a shorter way to write binary. See also: Binary, Denary.

I

IP address

A number that identifies a device on a network so that data can be sent to it. See also: Packet, Router.

Iteration

Repeating a set of instructions, using a loop such as for or while. See also: Selection, Sequence.

L

Linear search

A search that checks each item in turn from the start until the target is found or the list ends. See also: Binary search.

Logic error

A mistake that lets a program run but produces the wrong result. See also: Syntax error, Trace table.

Lossless compression

Compression that lets the original data be rebuilt exactly. See also: Lossy compression, Run-length encoding.

Lossy compression

Compression that permanently removes some data to make a file smaller, so the original cannot be rebuilt exactly. See also: Lossless compression.

M

Malware

Software designed to harm a computer or its user, such as a virus or ransomware. See also: Firewall, Phishing.

O

Operating system

Software that manages hardware, memory, files and users, and lets other programs run. See also: RAM.

Overflow error

An error that happens when a calculation produces a number too large to store in the available bits. See also: Binary, Bit.

P

Packet

A small chunk of data sent across a network, carrying a header with addressing information. See also: Router, Protocol.

Phishing

A scam in which someone pretends to be a trusted organisation, usually by email or text, to trick a victim into giving away information. See also: Social engineering.

Pixel

A single, smallest point of colour in a digital image. See also: Resolution.

Protocol

An agreed set of rules for how devices communicate. See also: Packet.

Pseudocode

A way of writing an algorithm in plain, structured statements that is not tied to one programming language. See also: Algorithm.

R

RAM (random access memory)

Volatile memory that holds the programs and data currently in use. See also: ROM, Volatile.

Register

A tiny, very fast storage location inside the CPU. See also: CPU, Fetch-decode-execute cycle.

Resolution

How many pixels an image has, often given as width by height. See also: Pixel.

ROM (read-only memory)

Non-volatile memory that holds start-up instructions and keeps them when the power is off. See also: RAM, Volatile.

Router

A device that forwards packets between networks. See also: Packet, IP address.

Run-length encoding

A lossless method that replaces a run of repeated values with a count and one copy of the value. See also: Lossless compression.

S

Sample rate

How many times per second a sound wave is measured when it is digitised. See also: Lossy compression.

Selection

Choosing which instructions to run depending on a condition, using if statements. See also: Iteration, Sequence.

Sequence

Instructions carried out one after another in the order they are written. See also: Selection, Iteration.

Social engineering

Manipulating people rather than machines to gain access or information. See also: Phishing.

SQL injection

An attack that enters database commands into an input field to read or change data without permission. See also: Validation.

String

A data type for text, made of a sequence of characters. See also: Data type, Casting.

Subroutine

A named block of code that can be called whenever it is needed, which avoids repeating code. See also: Function.

Syntax error

A mistake in the rules of the language, such as a missing bracket, that stops the program from running. See also: Logic error.

T

Trace table

A table that records the value of each variable as an algorithm runs, step by step. See also: Logic error, Algorithm.

U

Unicode

A character set that covers many more characters than ASCII, including other alphabets and emoji. See also: ASCII, Character set.

V

Validation

An automatic check that input is sensible, such as being within a range, without proving that it is correct. See also: SQL injection.

Variable

A named storage location whose value can change while the program runs. See also: Constant.

Volatile

Describes memory that loses its contents when the power is turned off. See also: RAM, ROM.

Part of the GCSE Computer Science revision set: the revision guide and self-marking quiz.