The Collaborative International Dictionary of English v.0.48:

Gyroscope \Gy"ro*scope\ (j[imac]"r[-o]*sk[=o]p), n. [Gr. gy^ros
   ring, circle + -scope.]
   1. A rotating wheel, mounted in a ring or rings, for
      illustrating the dynamics of rotating bodies, the
      composition of rotations, etc. It was devised by Professor
      W. R. Johnson, in 1832, by whom it was called the
      rotascope.
      [1913 Webster]

   2. A form of the above apparatus, invented by M. Foucault,
      mounted so delicately as to render visible the rotation of
      the earth, through the tendency of the rotating wheel to
      preserve a constant plane of rotation, independently of
      the earth's motion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gyroscope
    n 1: rotating mechanism in the form of a universally mounted
         spinning wheel that offers resistance to turns in any
         direction [syn: gyroscope, gyro]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "gyroscope":
   balance, balance rudder, balance wheel, ballast, centrifuge, chuck,
   counterbalance, counterweight, drill, extractor, fan, fin,
   fixative, flywheel, governor, hairspring, impeller, jack, keel,
   mordant, pendulum, propeller, reel, roller, rolling pin,
   rotary drill, rotor, screw, shock absorber, spindle, spinner, spit,
   spool, springs, stabilizator, stiffening, tail plane, teetotum,
   top, treadmill, turbine, turntable, whorl, windmill