sweatpants vs pair of trousers

sweatpants and pair of trousers both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweatpants	Yes	No	No	No
pair of trousers	Yes	No	No	No

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

sweatpants: 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 sweatpants include pant, trousers.

sweatpants (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