La polyrythmie consiste à superposer plusieurs rythmes d’accentuations différentes, par exemple binaires et ternaires. Chaque partie rythmique est appelée « motif » quand il s’agit d’un rythme qui se répète.

La notion de rythmique, et donc de polyrythmie, ne se limite pas à des parties de percussions.

source : Wikipedia

Polyrhythm is the simultaneous use of two or more conflicting rhythms, that are not readily perceived as deriving from one another, or as simple manifestations of the same meter. The rhythmic conflict may be the basis of an entire piece of music (cross-rhythm), or a momentary disruption. Polyrhythms can be distinguished from irrational rhythms, which can occur within the context of a single part; polyrhythms require at least two rhythms to be played concurrently, one of which is typically an irrational rhythm.

source : Wikipedia