vacuum flask vs flask

vacuum flask and flask both are nouns.

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

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

vacuum flask: flask with double walls separated by vacuum; used to maintain substances at high or low temperatures
flask: bottle that has a narrow neck

vacuum flask (noun)	flask (noun)
flask with double walls separated by vacuum; used to maintain substances at high or low temperatures	bottle that has a narrow neck
	the quantity a flask will hold