Rhythm is the measure of movement in time by regular recurring accents. When governed by rule, it is called metre. It is a major aspect of music, dance, and most poetry. The study of rhythm, stress, and pitch in speech is called prosody; it is a topic in linguistics.

In music, rhythms are usually arranged with respect to the time signature. The speed of a rhythm is called tempo. Beats which are emphasized by the time signature are called on beats; others are called off beats. In popular music, a strong off beat, called the beat or backbeat, usually repeats in the background, behind the melody (1234).

Some genres of music make different use of rhythm than others. Most western music is based on divisive rhythm, while non-western music uses more additive rhythm. African music makes heavy use of polyrhythms, and Indian music uses complex cycles such as 7 and 13, while Balinese music often uses complex interlocking rhythms. A lot of western classical music is rhythmically fairly simple; it stays in a simple meter such as 4/4 or 3/4 and makes little use of syncopation. In the 20th century, composers like Igor Stravinsky, Philip Glass, and Steve Reich wrote more rhythmically complex music using techniques such as phasing and additive rhythm. At the same time, modernists such as Olivier Messiaen and his pupils used increased complexity to disrupt the sense of a regular beat, leading eventually to the widespread use of irrational rhythms in New Complexity. LaMonte Young also wrote music in which the sense of a regular beat is absent because the music consists only of long sustained tones.

Instrumentalists who deal mainly with rhythm are called drummers or percussionists.

"Rhythm," wrote Tom Robbins in 'Another Roadside Attraction', "is everything pertaining to the duration of energy."

See also: riddim, musical notation, metre (music)