When two or more voices or instruments play the same rhythm but different notes, what is this practice called?

The practice of two or more voices or instruments playing the same rhythm but different notes is best described as homophonic texture. In homophony, while the voices may move in the same rhythm, they create distinct melodic lines, which adds richness and depth to the musical experience. This texture is foundational in many genres of music, providing a clear melodic line supported by harmonious accompaniment.

In contrast, harmony refers more broadly to the combination of different musical notes played or sung simultaneously, not necessarily implying the same rhythm. Imitation involves one voice or instrument echoing or repeating what another has played or sung, typically at a different time rather than simultaneously. Hocket is a technique where two voices alternate in a rhythmically interlocking manner, rather than maintaining a unified rhythmic pattern as seen in homophony. Thus, the characteristics of homophony align directly with the definition provided in the question.