from
The Free On-line Dictionary of Computing (8 July 2008)
tracker ball
track ball
<hardware> 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)