A function key is a key on a computer or terminal keyboard that can be programmed so as to cause an operating system command interpreter or application program to perform certain actions, a form of soft key. On some keyboards/computers, function keys may have default actions, accessible on power-on.

Function keys on a terminal may either generate short fixed sequences of characters, often beginning with the escape character (ASCII 27), or the characters they generate may be configured by sending special character sequences to the terminal. On a standard computer keyboard, the function keys may generate a fixed, single byte code, outside the normal ASCII range, which is translated into some other configurable sequence by the keyboard device driver or interpreted directly by the application program. Function keys may have abbreviations or pictographic representations of default actions printed on/besides them, or they may have the more common "F-number" designations.