overnighter vs overnight bag

overnighter and overnight bag both are nouns.

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

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

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

Other synonyms of overnighter include overnight case.

overnighter (noun)	overnight bag (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