Trackball

A trackball is 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, but with the ball sticking out more. The user rolls the ball with their thumb, fingers, or the palm of their hand to move a cursor. Trackballs were first seen in arcade games such as Missile Command and Centipede, but are now used to replace the mouse where space is limited.