bellbottom pants vs trousers

bellbottom pants and trousers both are nouns.

Word	Noun	Adjective	Verb	Adverb
bellbottom pants	Yes	No	No	No
trousers	Yes	No	No	No

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

bellbottom pants: (used in the plural) trousers with legs that flare; worn by sailors; absurdly wide hems were fashionable in the 1960s
trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

Other hypernyms of bellbottom pants include pair of trousers, pant.

bellbottom pants (noun)	trousers (noun)
(used in the plural) trousers with legs that flare; worn by sailors; absurdly wide hems were fashionable in the 1960s	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately