*servo-motor*

n. [ Sometimes erroneously spelt serro-motor. ] [ F. servo-moteur. See Serf; Motor. ] 1. (Mach.) A relay apparatus; specif.: (a) An auxiliary motor, regulated by a hand lever, for quickly and easily moving the reversing gear of a large marine engine into any desired position indicated by that of the hand lever, which controls the valve of the motor. (b) In a Whitehead torpedo, a compressed-air motor, for moving the rudders so as to correct deviations from the course. [ Webster 1913 Suppl. ]

2. a motor which forms part of a servomechanism. [ PJC ]