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]