The Collaborative International Dictionary of English v.0.48:

Metronome \Met"ro*nome\, n. [Gr. ? measure + ? distribute,
   assign: cf. F. m['e]tronome, It. metronomo.]
   An instrument consisting of a short pendulum with a sliding
   weight. It is set in motion by clockwork, and serves to
   measure time in music.
   [1913 Webster]

WordNet (r) 3.0 (2006):

metronome
    n 1: clicking pendulum indicates the exact tempo of a piece of
         music