By Matt Byonick

The subdivision of a span of time into perceptible sections; the grouping of musical sounds, principally by means of duration and stress.
In Western music, time is usually organized to establish a regular pulse, and by the subdivision of that pulse into regular groups. Such groups are commonly of two or three units (or their compounds, such as four or six), the arrangement of the pulse into groups is the metre of a composition, and the rate of pulses is its tempo. Most Western music, from the late Middle Ages to the 20th century, possesses a regular rhythmic pulse and metre; these may be absent, however, from some types of earlier music, for example ecclesiastical plainchant, which apparently lacked a metric structure, leaving its rhythm to be realized according to conventions and as dictated by the verbal text. In some 20th-century music, composers have sought to avoid regular rhythmic structures in order to achieve a more flexible rhythm; in some cases their methods have been influenced by folk music lacking a regular metric structure. But even in periods when composers accepted 'the tyranny of the bar-line', they have used various devices to prevent dull or sterile rhythmic structures: syncopation (displacement of accent); shorter notes at stressed parts of the bar (as opposed to the more natural longer notes at accented points); phrases that avoid regular four-bar or eight-bar patterns; eliding phrases into one another or extending phrases; short-term displacement of accent (common in cadences in the Baroque era: see hemiola); in vocal music, following a natural speech rhythm (either one that contradicts metric regularity, as in much recitative, or one that follows a broad metric structure created by a verbal text).
Rhythm, as a fundamental element - no music can exist other than in time - has a part to play in many other aspects of music: it is an important element in melody, it affects the progression of harmony, and has a role in such matters as texture, timbre and ornamentation. It is fundamental to the dance; dance patterns, derived from natural rhythms of bodily motion, have dictated many of the rhythmic patterns that pervade Westem music.
While in Westem music rhythm is multiplicative (i.e. rhythmic pattems are derived by multiplying or dividing, normally by two or three), in many non-Western cultures they are additive; an eight-unit rhythm in Western music is invariably constructed on the basis 2 x 2 x 2, in Middle Eastem music it may well be 3 + 2 + 3.