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

tracker ball
track ball

    A pointing device consisting of a ball housed in
   a socket containing sensors to detect rotation of the ball
   about two axes - like an upside-down mouse.  The user rolls
   the ball with his thumb or the palm of his hand to move a
   cursor.  Tracker balls are common on CAD workstations for
   ease of use and on modern portable computers, where there
   may be no desk space on which to use a mouse.  Some clip
   onto the side of the keyboard and have integral buttons which
   have the same function as mouse buttons.

   (1996-08-04)