plain flour vs flour

plain flour and flour both are nouns.

Word	Noun	Adjective	Verb	Adverb
plain flour	Yes	No	No	No
flour	Yes	No	Yes	No

As nouns, flour is a hypernym of plain flour; that is, flour is a word with a broader meaning than plain flour:

plain flour: flour that does not contain a raising agent
flour: fine powdery foodstuff obtained by grinding and sifting the meal of a cereal grain

plain flour (noun)	flour (noun)
flour that does not contain a raising agent	fine powdery foodstuff obtained by grinding and sifting the meal of a cereal grain

plain flour (verb)	flour (verb)
	convert grain into flour
	cover with flour