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1550s, "rhymed verse, metrical movement," from Latin rhythmus "movement in time," from Greek rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement, order; form, shape, wise, manner; soul, disposition," related to rhein "to flow," from PIE root *sreu- "to flow" (see rheum). Rhythm method of birth control attested from 1936. Rhythm and blues, U.S. music style, is from 1949 (first in "Billboard").
Movement or variation characterized by the regular recurrence or alternation of different quantities or conditions, as in the heartbeat.