In his motets Machaut strove for all-over patterns of rhythmic unity through isorhythms, a system codifying a few set patterns of time value, the number of repetitions of a pattern within a composition depending upon the desired length […]
Source: wiktionary
Even when such preconceived structural arrangements are not used, as in the Gloria of the Machaut Mass, the idea of isorhythm is present in the suggestion of recurring and balancing strophic plans.
Source: wiktionary
Meaning simply "same rhythm," isorhythm refers to the practice of using the same rhythmic pattern, called in fourteenth-century treatises a talea ("cutting"), over and over in one or more part as the structural base for a motet or mass movement.
Source: wiktionary