long pants vs trousers

long pants and trousers both are nouns.

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

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

long pants: trousers reaching to the foot
trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

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

long pants (noun)	trousers (noun)
trousers reaching to the foot	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately