sweat pants vs pair of trousers

sweat pants and pair of trousers both are nouns.

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

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

sweat pants: loose-fitting trousers with elastic cuffs; worn by athletes
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 sweat pants include pant, trousers.

sweat pants (noun)	pair of trousers (noun)
loose-fitting trousers with elastic cuffs; worn by athletes	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately