Longman Dictionary of Contemporary English
plain flour
noun
EXAMPLES FROM CORPUS
▪ Appendix B Recipes for play dough Mix plain flour and water with a little salt.
▪ Thicken towards the end of cooking if necessary with plain flour mixed with a little water and a few drops of bottled gravy browning.
Wiktionary
plain flour
n. (context US standard of identity English) (form of exact synonym flour English)
WordNet
plain flour
n. flour that does not contain a raising agent