Polyrhythm – A combination of two or more rhythms played simultaneously while moving at the same tempo/pulse/beat

We know what poly means and we know what rhythm means.

Poly - Many

And

Rhythm - A pattern of sounds of different lengths

So Polyrhythm must be two or more rhythms (played simultaneously).

The following is an example of a simple polyrhythm with 3 being played against 4.

The following is an example of a simple polyrhythm with 3 being played against 4