Which is the correct term for music that features multiple differing rhythms going on at the same time?

The correct term for music that features multiple differing rhythms occurring simultaneously is "polyrhythmic." Polyrhythm refers to the use of two or more contrasting rhythms played together, creating a complex and layered rhythmic texture. This technique is often found in various musical traditions, such as African, Indian, and some modern Western music. It can involve different rhythmic patterns that may not necessarily conform to a common meter, resulting in rich and intricate musical experiences.

In contrast, terms like "back-beat" refer to a rhythmic emphasis usually found in popular music on the second and fourth beats of a measure, which is quite different from the idea of layering multiple distinct rhythms. "Mixed meter" involves the use of different time signatures within a piece but does not inherently imply the simultaneous presentation of contrasting rhythms. "Syncopated" refers to a rhythmic effect created by placing emphasis on normally weak beats or offbeats, which results in a shift in the expected rhythmic pattern but does not involve multiple conflicting rhythms happening at once. Therefore, the identification of "polyrhythmic" correctly describes the complexity of having differing rhythms together in a musical composition.