Source : Free On-Line Dictionary of Computing

function key
     
         (From the {IBM 3270} terminal's Programmed Function
        Keys (PF keys)) One of a set of special keys on a computer or
        {terminal} keyboard which can be programmed so as to cause an
        {application program} to perform certain actions.
     
        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 {microcomputer} 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}.
     
        (1995-02-07)