Wordnet / HyperDictionary Definition

1. Recurring at regular intervals
2. The arrangement of spoken words alternating stressed and unstressed elements; "the rhythm of Frost's poetry"
3. The basic rhythmic unit in a piece of music; "the piece has a fast rhythm"; "the conductor set the beat"
4. An interval during which a recurring sequence of events occurs; "the neverending cycle of the seasons"

Wikipedia Definition

Rhythm (Greek ρυθμός = tempo) is the variation of the duration of sounds over time. When governed by rule, it is called meter. It is inherent in any time-dependent medium, but it is most associated with music, dance, and the majority of poetry. The study of rhythm, stress, and pitch in speech is called prosody; it is a topic in linguistics. All musicians, instrumentalists and vocalists, work with rhythm, but it is often considered the primary domain of drummers and percussionists.

In Western music, rhythms are usually arranged with respect to a time signature, partially signifying a meter. The speed of the underlying pulse, called the beat, is the tempo. The tempo is usually measured in 'beats per minute' (bpm); 60 bpm means a speed of one beat per second. The length of the meter, or metric unit (usually corresponding with measure length), is divided almost exclusively into either two or three beats, being called duple meter and triple meter, respectively. If each beat is further divided by two it is simple meter, if by three compound meter.

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. By comparison, a lot of Western classical music is fairly rhythmically 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 odd meters, and 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 (drones).

Clave is a common underlying rhythm in African, Cuban music, and Brazilian music.

A rhythm section generally consists of percussion instruments, and possibly chordal instruments (e.g., guitar, banjo) and keyboard instruments, such as piano (which, by the way, may be classified in any of these three types of instruments).

Narmour (1980, p.147-53) describes three categories of prosodic rules which create rhythmic successions which are additive (same duration repeated), cumulative (short-long), or countercumulative (long-short). Cumulation is associated with closure or relaxation, countercumulation with openness or tension, while additive rhythms are open-ended and repetitive. Richard Middleton points out this method cannot account for syncopation and suggests the concept of transformation.

A rhythmic unit is a durational pattern which occupies a period of time equivalent to a pulse or pulses on an underlying metric level, as opposed to a rhythmic gesture which does not.