The Free On-line Dictionary of Computing (30 December 2018):

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)