nightwear vs pajama

nightwear and pajama both are nouns.

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

As nouns, pajama is a hyponym of nightwear; that is, pajama is a word with a more specific, narrower meaning than nightwear:

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

Other hyponyms of nightwear include gown, night-robe, nightdress, nightgown, nightie, nightshirt, jammies, pj's, pyjama.

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