pair of trousers vs trousers

pair of trousers and trousers both are nouns.

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

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

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

Other synonyms of pair of trousers include pant.

pair of trousers (noun)	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