Polyrhythm

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.

In some European art music, polyrhythm periodically contradicts the prevailing meter. For example, polyrhythm is heard near the opening of Beethoven's Third Symphony. (See also syncopation.) It is a particularly common feature of the music of Brahms. Writing about the Violin Sonata in G major, Op.78, Jan Swafford (1997, p. 456) says “In the first movement Brahms plays elaborate games with the phrasing, switching the stresses of the 6/4 meter back and forth between 3+3 and 2+2+2, or superimposing both in violin and piano…. These ideas gather at the climax at measure 235, with the layering of phrases making an effect that perhaps during the 19th century only Brahms could have conceived."

Concerning the use of a two-over-three (2:3) hemiola in Beethoven's Sixth String Quartet, Ernest Walker states, "The vigorously effective Scherzo is in 3
4 time, but with a curiously persistent cross-rhythm that does its best to persuade us that it is really in 6
8."

...
Wikipedia