long trousers vs trousers

long trousers and trousers both are nouns.

Word	Noun	Adjective	Verb	Adverb
long trousers	Yes	No	No	No
trousers	Yes	No	No	No

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

long trousers: trousers reaching to the foot
trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

Other hypernyms of long trousers include pair of trousers, pant.

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