Source : Free On-Line Dictionary of Computing

tracker 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)