relation vs better half

relation and better half both are nouns.

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

As nouns, better half is a hyponym of relation; that is, better half is a word with a more specific, narrower meaning than relation:

relation (noun)	better half (noun)
an abstraction belonging to or characteristic of two entities or parts together	a person's partner in marriage
(usually plural) mutual dealings or connections among persons or groups
sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur
(law) the principle that an act done at a later time is deemed by law to have occurred at an earlier time
an act of narration
a person related by blood or marriage