canteen vs flask

canteen and flask both are nouns.

Word	Noun	Adjective	Verb	Adverb
canteen	Yes	No	No	No
flask	Yes	No	No	No

As nouns, flask is a hypernym of canteen; that is, flask is a word with a broader meaning than canteen:

canteen (noun)	flask (noun)
a flask for carrying water; used by soldiers or travelers	bottle that has a narrow neck
restaurant in a factory; where workers can eat	the quantity a flask will hold
a recreation room in an institution
a restaurant outside; often for soldiers or policemen
sells food and personal items to personnel at an institution or school or camp etc.