Two or more contrasting rhythms played simultaneously; two or more rhythms superimposed.

Systematic exploitation of several rhythms performed simultaneously.

rhythm laid on top of (or beneath) a related or contrasting rhythm (or rhythms)

Polyrhythm is the simultaneous sounding of two or more independent rhythms. 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.