bikini pants vs underpants

bikini pants and underpants both are nouns.

Word	Noun	Adjective	Verb	Adverb
bikini pants	Yes	No	No	No
underpants	Yes	No	No	No

As nouns, underpants is a hypernym of bikini pants; that is, underpants is a word with a broader meaning than bikini pants:

bikini pants: (used in the plural) small and tight-fitting underpants; worn by women
underpants: an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin

bikini pants (noun)	underpants (noun)
(used in the plural) small and tight-fitting underpants; worn by women	an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin