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- hacker
Someone experienced in attempting to gain unauthorised access into computer systems. 
- hacking
Attempting (with or without success) to gain unauthorised access to a computer system. This may be the unauthorised use of a computer or simply unauthorised access to particular programs or data stored on a computer. - half duplex
Refers to the direction of data transmission. It may be in both directions at the same time, called duplex or full duplex; in only one direction at a time, called half duplex; or in one direction only, called simplex. - hand-held scanner
A scanner is a device that can detect and copy an image, using a beam of light, which then is stored in a computer. With a hand-held scanner, the user has to move the scanner by hand over the document. - handshake
The exchange of signals between devices to establish their readiness to send or receive data, e.g. the transmission of data from a computer to a printer. Handshaking is one of a number of methods of ensuring that both the sender and receiver are ready before transmission begins. - handwriting recognition
The analysis of handwritten input, comparing the result with samples already stored in the computer and turning it into characters which can be processed by the computer. In simpler systems the user has to write in separate characters. Although some software can recognise cursive (joined-up) writing it needs greater processing power and the system has to be - hang
The term used to describe the situation when a computer system unexpectedly stops working, and does not respond to inputs. - hard copy
Information from a computer source printed on paper. - hard disk
A fixed disk, normally non-removable, in a computer. Hard disks store more data and can be accessed much more quickly than floppy disks. - hard drive
The unit made up of the mechanism that rotates the disks between the read/write heads and the mechanism that controls the heads. The hard drive incorporates a fixed disk, usually non-removable. - hard-wired
Unable to be changed, a function permanently built into the circuitry. Often this is an integrated circuit designed to control the function. Such functions are immediately available when switched on and cannot be altered by the user. This ensures that the device will always be in the same initial state when switched on. - hardware
The hardware is the equipment used in a computer system, for example, central processing unit, keyboard, monitor, mouse, joystick and printer. - hash
The calculation of a numeric value from one or more data items. - hash table
A hash table is one way of structuring data. The data is stored in a table, and a mathematical calculation (hash) is used on one field of the data, to provide its location within the table. This means that the data can be accessed directly, without having to go through the entire table searching for it. - head crash
A head crash is when the read/write heads hit the surface of a disk. This can cause serious damage to the mechanisms and loss of data. To avoid this happening with personal computers, e.g. when a computer is moved, the read/write heads are often automatically put in a safe position (parked). - header
Text that appears at the top of every page in a document. - headphones
A device to plug into a computer (or other device) to listen to audio. - headset
A combination of headphones and a microphone to record and listen to audio, and to communicate with others. - height
The size of something from top to bottom. Used, for example, in graphics software, where the user needs to specify the height and width of an image. Also in spreadsheets, etc., where the height of cells and tables can be changed. Usually measured in pixels, inches, or centimetres. - help
A feature of most programs, to help the user when they need more information on how to use the program, or particular features within it.
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