ritmo

Rhythm and rimo are the same Greek word, split in two on the way here, and Esperanto imported both halves as separate roots. So the ancient relationship is genuine and completely useless for building either one from the other. Ritmo is the beat, the pattern of stressed and unstressed syllables; rimo is only the matching of sounds at the ends of lines.

Esperanto has an unusual rhythm problem, and it is the mirror image of its rhyme problem. Stress is fixed on the second-to-last syllable of every word, without exception. That makes the metre wonderfully predictable to read and unusually hard to write: the poet cannot shift a stress to save a line, so the whole burden falls on choosing words of the right length. Elision helps, which is why l' exists at all. The full picture is in poetic Esperanto, which needs this word to say any of it.

Outside poetry it is the ordinary international noun. Ritma is rhythmic, ritme rhythmically, senritma without rhythm. It carries the everyday metaphors too: la ritmo de la laboro, la ritmo de la tagoj, the pace at which a thing recurs.