bigamist vs better half

bigamist and better half both are nouns.

Word	Noun	Adjective	Verb	Adverb
bigamist	Yes	No	No	No
better half	Yes	No	No	No

As nouns, better half is a hypernym of bigamist; that is, better half is a word with a broader meaning than bigamist:

bigamist: someone who marries one person while already legally married to another
better half: a person's partner in marriage

Other hypernyms of bigamist include married person, mate, partner, spouse.

bigamist (noun)	better half (noun)
someone who marries one person while already legally married to another	a person's partner in marriage