trousers vs pair of trousers

trousers and pair of trousers both are nouns.

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

As nouns, trousers and pair of trousers are synonyms defined as:

trousers and pair of trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

Other synonyms of trousers include pant.

trousers (noun)	pair of trousers (noun)
(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately