denim vs pair of trousers

denim and pair of trousers both are nouns.

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

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

denim: (used in the plural) close-fitting trousers of heavy denim for manual work or casual wear
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 denim include pant, trousers, workwear.

denim (noun)	pair of trousers (noun)
a coarse durable twill-weave cotton fabric	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately
(used in the plural) close-fitting trousers of heavy denim for manual work or casual wear