The Collaborative International Dictionary of English v.0.48:

Servomechanism \Ser`vo*mech"an*ism\, n.
   an electronically controlled mechanical or hydraulic device
   permitting a large action or strong forces to be controlled
   by a small electrical signal.
   [PJC]

WordNet (r) 3.0 (2006):

servomechanism
    n 1: control system that converts a small mechanical motion into
         one requiring much greater power; may include a negative
         feedback system [syn: servo, servomechanism,
         servosystem]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "servomechanism":
   Flettner control, action, autosyn, clockworks, cybernion,
   direct-current servo, drive train, gear, hemostat, hydraulic servo,
   innards, machinery, mechanism, motion, movement, movements,
   pneumatic servo, power brake, power servo, power train, selsyn,
   servo, servo brake, servomotor, servotab, sine mechanism, synchro,
   synchro receiver, synchro transmitter, synchronous motor,
   telemechanism, watchworks, wheels, wheels within wheels, workings,
   works