in music, the organizing principle of much of 14th-century French polyphony, characterized by the extension of the rhythmic texture (talea) of an initial section to the entire composition, despite the variation of corresponding melodic features (color); the term was coined around 1900 by the German musicologist Friedrich Ludwig.

A logical outgrowth of the rhythmic modes (fixed patterns of triple rhythms) that governed most late medieval polyphony, isorhythm first appeared in 13th-century motets, primarily in cantus firmus or tenor parts but occasionally in other voices as well. Abandoning all modal limitations, the isorhythmic motet of the 14th century managed to derive decisive structural benefit from the systematic application of given rhythmic patterns without the inescapable dance associations of its 13th-century predecessor. The first great master of the isorhythmic motet was Guillaume de Machaut (c. 1300–77), but instances of isorhythm occurred as late as the early work of the 15th-century Burgundian composer Guillaume Dufay (c. 1400–74). As an analytical concept, isorhythm has proved valuable in connection with musical practices quite unrelated to those of the European Middle Ages—for example, peyote cult songs of certain North American Indian groups.