pyjama vs pj's

pyjama and pj's both are nouns.

Word	Noun	Adjective	Verb	Adverb
pyjama	Yes	No	No	No
pj's	Yes	No	No	No

As nouns, pyjama and pj's are synonyms defined as:

pyjama and pj's: (usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers

Other synonyms of pyjama include jammies, pajama.

pyjama (noun)	pj's (noun)
(usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers	(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