pajama vs nightclothes

pajama and nightclothes both are nouns.

Word	Noun	Adjective	Verb	Adverb
pajama	Yes	No	No	No
nightclothes	Yes	No	No	No

As nouns, nightclothes is a hypernym of pajama; that is, nightclothes is a word with a broader meaning than pajama:

pajama: (usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers
nightclothes: garments designed to be worn in bed

Other hypernyms of pajama include nightwear, sleepwear.

pajama (noun)	nightclothes (noun)
(usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers	garments designed to be worn in bed
a pair of loose trousers tied by a drawstring around the waist; worn by men and women in some Asian countries