rhythm vs offbeat

rhythm and offbeat both are nouns.

Word	Noun	Adjective	Verb	Adverb
rhythm	Yes	No	No	No
offbeat	Yes	Yes	No	No

As nouns, offbeat is a hyponym of rhythm; that is, offbeat is a word with a more specific, narrower meaning than rhythm:

Other hyponyms of rhythm include backbeat, downbeat, upbeat, syncopation, free rhythm, measured rhythm.

rhythm (noun)	offbeat (noun)
natural family planning in which ovulation is assumed to occur 14 days before the onset of a period (the fertile period would be assumed to extend from day 10 through day 18 of her cycle)	an unaccented beat (especially the last beat of a measure)
recurring at regular intervals
(architecture) the repetitive use of a group of visual elements to establish a recognizable pattern
the arrangement of spoken words alternating stressed and unstressed elements
the basic rhythmic unit in a piece of music
an interval during which a recurring sequence of events occurs

rhythm (adjective)	offbeat (adjective)
	(informal) strikingly unconventional