septet vs heptad

septet and heptad both are nouns.

Word	Noun	Adjective	Verb	Adverb
septet	Yes	No	No	No
heptad	Yes	No	No	No

As nouns, septet and heptad are synonyms defined as:

Other synonyms of septet include 7, septenary, seven, sevener, VII.

septet (noun)	heptad (noun)
a musical composition written for seven performers	the cardinal number that is the sum of six and one
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