overnight case vs overnight bag

overnight case and overnight bag both are nouns.

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

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

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

Other synonyms of overnight case include overnighter.

overnight case (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