Polyrhythms are the simultaneous occurrence of two or more sharply contrasting rhythms within a music composition.
One of the most common polyrhythms is the 3:2 cross-rhythm. A cross-rhythm is a continuous polyrhythm where the pattern of accents of the prevailing meter is contradicted by a conflicting pattern and not merely a momentary displacement that leaves the prevailing meter fundamentally unchallenged. The 3:2 cross-rhythm is the foundation of most West African music.
Typically, two rhythms will only be considered a polyrhythm if they have no common divisor other than 1. However, both rhythms can be multiplied together to create a single composite rhythm.