octet vs piece of music

octet and piece of music both are nouns.

Word	Noun	Adjective	Verb	Adverb
octet	Yes	No	No	No
piece of music	Yes	No	No	No

As nouns, piece of music is a hypernym of octet; that is, piece of music is a word with a broader meaning than octet:

Other hypernyms of octet include composition, musical composition, opus, piece.

octet (noun)	piece of music (noun)
a musical composition written for eight performers	a musical work that has been created
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