underclothing vs undergarment

underclothing and undergarment both are nouns.

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

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

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

Other hypernyms of underclothing include unmentionable.

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