short pants vs pair of trousers

short pants and pair of trousers both are nouns.

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

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

short pants: (used in the plural) trousers that end at or above the knee
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 short pants include pant, trousers.

short pants (noun)	pair of trousers (noun)
(used in the plural) trousers that end at or above the knee	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately