WordNet (r) 3.0 (2006):

control key
    n 1: (computer science) the key on a computer keyboard that is
         used (in combination with some other key) to type control
         characters [syn: control key, command key]

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

control key

    A modifier key found on modern keyboards.

   Holding down a control key while pressing and releasing letter
   keys or certain other keys generates a "control character".
   E.g. holding control and hitting "A" generates control-A (ASCII
   code 1).  The ASCII code for the control character is generally 64
   less than that for the unmodified character.

   Standard PC keyboards have two control keys, both labeled
   "Ctrl", at the bottom left and bottom right of the main block of
   keys.

   The control key does not generate any character on its own but
   most modern keyboards and operating systems allow a program
   to tell whether each of the individual keys on the keyboard
   (including modifier keys) is pressed at any time.

   (2015-03-07)