septet vs piece of music

septet and piece of music both are nouns.

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

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

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

septet (noun)	piece of music (noun)
a musical composition written for seven performers	a musical work that has been created
seven people considered as a unit
a set of seven similar things considered as a unit
seven performers or singers who perform together
the cardinal number that is the sum of six and one