A trackball is a pointing device consisting of a ball held by a socket containing sensors to detect a rotation of the ball about two axes—like an upside-down mouse with an exposed protruding ball. The user rolls the ball with the thumb, fingers, or the palm of the hand to move a pointer.
英英词典
track ball n.
an electronic device consisting of a rotatable ball in a housing; used to position the cursor and move images on a computer screen