Open English Wordnet
(n)rotary encoder, shaft encoderan electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.“Rotary encoders are used in many applications that require precise shaft unlimited rotation—including industrial controls, robotics, special purpose photographic lenses, computer input devices (such as optomechanical mice and trackballs), controlled stress rheometers, and rotating radar platforms.”