underclothes vs undergarment

underclothes and undergarment both are nouns.

Word	Noun	Adjective	Verb	Adverb
underclothes	Yes	No	No	No
undergarment	Yes	No	No	No

As nouns, undergarment is a hypernym of underclothes; that is, undergarment is a word with a broader meaning than underclothes:

underclothes: undergarment worn next to the skin and under the outer garments
undergarment: a garment worn under other garments

Other hypernyms of underclothes include unmentionable.

underclothes (noun)	undergarment (noun)
undergarment worn next to the skin and under the outer garments	a garment worn under other garments