Corn flour
Finely ground yellow maize, a flour with a mild, slightly sweet corn flavour; not to be confused with pure cornstarch, which British English also calls cornflour.
Fine corn flour is sold in well-stocked supermarkets, health food shops, Turkish or Latin American grocers and by mail order; polenta and cornmeal are coarser. In Britain the label cornflour usually means cornstarch, a white, flavourless powder that behaves differently, so check that the bag contains whole ground maize.
Substitute:
Fine cornmeal or polenta ground further in a blender comes closest, though it stays slightly grittier.