octet vs octad

octet and octad both are nouns.

Word	Noun	Adjective	Verb	Adverb
octet	Yes	No	No	No
octad	Yes	No	No	No

As nouns, octet and octad are synonyms defined as:

Other synonyms of octet include 8, eight, eighter, eighter from Decatur, octonary, ogdoad, VIII.

octet (noun)	octad (noun)
a musical composition written for eight performers	the cardinal number that is the sum of seven and one
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