overnight bag vs suitcase

overnight bag and suitcase both are nouns.

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

As nouns, suitcase is a hypernym of overnight bag; that is, suitcase is a word with a broader meaning than overnight bag:

overnight bag: a small traveling bag to carry clothing and accessories for staying overnight
suitcase: a portable rectangular container for carrying clothes

Other hypernyms of overnight bag include bag, grip, traveling bag, travelling bag.

overnight bag (noun)	suitcase (noun)
a small traveling bag to carry clothing and accessories for staying overnight	a portable rectangular container for carrying clothes