An input device that looks like an upside down mouse. The onscreen pointer is moved by the trackball with a thumb or finger.
A trackball requires less arm and wrist motion that a regular mouse takes and therefore is often less stressful for the user to use, helping to prevent RSI. The picture of the Logitech cordless trackball mouse is an example of a trackball mouse that uses the thumb for movement.
The first trackball device was invented by Ralph Benjamin in 1946. Named the "roller ball," it was designed for use with a military radar plotting system. A prototype device was developed, and its use was a military secret, never released to the public.