Real Dictionary

metronome
countable noun: A metronome is a device which is used to indicate how quickly a piece of music should be played. It can be adjusted to make regular sounds at different speeds.

metronome in American English
a clockwork device with an inverted pendulum that beats time at a rate determined by the position of a sliding weight on the pendulum: it is used esp. to help a person maintain regular tempo in practicing on the piano, etc.
noun: a mechanical or electrical instrument that makes repeated clicking sounds at an adjustable pace, used for marking rhythm, esp. in practicing music

metronome in British English
noun: a mechanical device which indicates the exact tempo of a piece of music by producing a clicking sound from a pendulum with an adjustable period of swing