A metronome is any device that produces regular, metrical ticks (beats, clicks) — settable in beats per minute. These ticks represent a fixed, regular aural pulse; some metronomes also include synchronized visual motion (e.g. pendulum-swing). The metronome dates from the early 19th century, where it was patented by Johann Maelzel in 1815 as a tool for musicians, under the title "Instrument/Machine for the Improvement of all Musical Performance, called Metronome".

The metronome is used by musicians to help keep a steady tempo as they play, or to work on issues of irregular timing, or to help internalize a clear sense of timing and tempo. The metronome is also often used by composers as a standard tempo reference, to indicate the intended tempo for the piece.

Human beings seldom play music at an exact tempo with all the beats exactly the same. This makes it impossible to align metronome clicks with the beats of a musically expressive performance. This also has lead many musicians to criticize use of a metronome. Some go as far as to suggest that metronomes shouldn't be used by musicians at all. The same criticism has been applied to metronome markings as well. See Criticism of metronome use.

Those in favour of metronome use understand this as a criticism of metronome technique as commonly practised by musicians, rather than criticism of the tool as such. Their response has been to develop better methods of metronome technique to address the various issues raised by the critics. See Metronome Technique. Frederick Franz expressed it well in his book, and what he says still applies today (the original version was published in 1947)

There are two schools of thought among musicians concerning this use of the metronome-one opposed and the other favorable. "Practicing with a metronome" has been criticized by some musicians as "making you mechanical." In some instances such criticism is largely a prejudice, the critic having gained the impression that one starts a metronome and simply continues playing with it indefinitely. In most instances, however, such criticism is excusable since so little has been published on specific techniques of metronome uses. It is hoped that those who oppose its use for learning and improving the control of rhythm will read with tolerance these methods, employed by those who favor it, and perhaps investigate their value by experimenting with one or two of them in their own teaching or preparation for concerts.

Read more about Metronome:  Etymology, History, Usage, Standard Appearance, Types of Metronomes, Use of The Metronome As An Instrument, Metronome Technique