overnighter vs overnight case

overnighter and overnight case both are nouns.

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

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

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

Other synonyms of overnighter include overnight bag.

overnighter (noun)	overnight case (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