musical group vs octet

musical group and octet both are nouns.

Word	Noun	Adjective	Verb	Adverb
musical group	Yes	No	No	No
octet	Yes	No	No	No

As nouns, octet is a hyponym of musical group; that is, octet is a word with a more specific, narrower meaning than musical group:

musical group (noun)	octet (noun)
an organization of musicians who perform together	a musical composition written for eight performers
	eight people considered as a unit
	a set of eight similar things considered as a unit
	eight performers or singers who perform together
	the cardinal number that is the sum of seven and one