flour vs plain flour

flour and plain flour both are nouns.

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

As nouns, plain flour is a hyponym of flour; that is, plain flour is a word with a more specific, narrower meaning than flour:

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

Other hyponyms of flour include wheat flour, soy flour, soybean flour, soybean meal, semolina, rye flour.

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

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