long pants vs pair of trousers

long pants and pair of trousers both are nouns.

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

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

long pants: trousers reaching to the foot
pair of 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 pant, trousers.

long pants (noun)	pair of 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