masa harina
Finely ground flour made from corn that has first been cooked in lime water. Mixed with water, it becomes the dough for tortillas and tamales.
Masa harina is masa dough that has been dried and ground. Corn kernels are cooked and soaked in lime water, a process called nixtamalization, then ground and dried. This treatment changes the corn so that the flour forms a pliable, cohesive dough with water that can be pressed and cooked. It tastes distinctly different from ordinary cornmeal, earthy, slightly mineral and recognisably of tortillas. It is sold in Latin American grocery shops, in some well-stocked supermarkets and by mail order; in North America in particular it is often on the regular supermarket shelf.
Substitute:
There is no good substitute. Cornmeal, corn flour, grits or polenta have not been nixtamalized, so they do not form a cohesive dough and crumble when shaped into tortillas; the characteristic flavour is missing as well. Fresh masa, where available, can be used instead.