Source : Webster's Revised Unabridged Dictionary (1913)

Servo-motor \Ser`vo-mo"tor\, n. [Sometimes erroneously spelt
   serro-motor.] [F. servo-moteur. See {Serf}; {Motor}.] (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.