From Longman Dictionary of Contemporary Englishtrackballtrack‧ball /ˈtrækbɔːl $ -bɒːl/ noun [countable] TDa small ball connected to a computer, that you turn in order to move the cursor
Examples from the Corpus
trackball• The reason that conventional trackballs and joysticks aren't used is because the precise positioning that they provide simply isn't needed.• The icons are generally activated by an on-screen pointer controlled by a mouse or trackball.• Klatsky found that some subjects preferred trackballs or Glidepoint pads even though they actually performed better with mice. 