underclothing vs BVD

underclothing and BVD both are nouns.

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

As nouns, BVD is a hyponym of underclothing; that is, BVD is a word with a more specific, narrower meaning than underclothing:

underclothing: undergarment worn next to the skin and under the outer garments
BVD: trademark for men's underwear

Other hyponyms of underclothing include BVD's, intimate apparel, lingerie, long johns, skivvies, tighty whitey.

underclothing (noun)	BVD (noun)
undergarment worn next to the skin and under the outer garments	trademark for men's underwear