overnight bag vs overnighter

overnight bag and overnighter both are nouns.

Word	Noun	Adjective	Verb	Adverb
overnight bag	Yes	No	No	No
overnighter	Yes	No	No	No

As nouns, overnight bag and overnighter are synonyms defined as:

overnight bag and overnighter: a small traveling bag to carry clothing and accessories for staying overnight

Other synonyms of overnight bag include overnight case.

overnight bag (noun)	overnighter (noun)
a small traveling bag to carry clothing and accessories for staying overnight	a small traveling bag to carry clothing and accessories for staying overnight
	a guest who stays overnight