slacks vs pair of trousers

slacks and pair of trousers both are nouns.

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

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

slacks: (used in the plural) pants for 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 slacks include pant, trousers.

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