Any of the 10 or more keys labeled F1, F2, F3, and so on, that are placed along the left side or across the top of a keyboard (or both) and are used for special tasks by different programs. The meaning of a function key is defined by a program or, in some instances, by the user. Function keys are used in application programs or the operating system to provide either a shortcut for a series of common instructions (such as calling up a program’s on-screen help facility) or a feature that is not otherwise available. See also key (definition 1). Compare Command key, Control key, Escape key. Key on a keyboard that, when pressed, performs a designated task, such as ending a computer program.