sweat pants vs sweatpants

sweat pants and sweatpants both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweat pants	Yes	No	No	No
sweatpants	Yes	No	No	No

As nouns, sweat pants and sweatpants are synonyms defined as:

sweat pants and sweatpants: loose-fitting trousers with elastic cuffs; worn by athletes

sweat pants (noun)	sweatpants (noun)
loose-fitting trousers with elastic cuffs; worn by athletes	loose-fitting trousers with elastic cuffs; worn by athletes