Masa de Harina.
Masa Harina literally means dough flour in Spanish and in Mexican cuisine, it refers to flour made from maize that has been soaked in lime water (calcium hydroxide). It is not the same thing as cornmeal and cannot be used interchangeably. The process of soaking the maize in lime water softens the kernels changing the texture so that the finished dough is more elastic and workable. After the maize is soaked, it's ground then used fresh as masa, or dried to make masa harina.
In the photo above you can see the fine almost white powder on the right with the dough it forms on the left. In the back is a bag of Maseca brand Masa Harina.
What's it taste like?
It has a nutty slightly minerally flavour that unsurprisingly tastes like corn.
Where do I get it?
Grocery stores in North America that have a Latin American food section should carry it. Otherwise go to a Latin American specialty food store. It typically comes in bags that look like a bag of flour. Personally I like the Maseca brand.
When is it best?
There's no season, but if you are able to find fresh masa, which looks more like a dough, the flavour and texture are better than rehydrating dried masa harina.
How do I use it?
Masa harina can be worked into a dough by adding water then allowing it to rest for about an hour to fully rehydrate. This dough can then be pressed into corn tortillas which can be "baked" on a hot cast iron skillet. These tortilla's can then be used to make tacos or enchiladas or just served along side a stew. If they are cut and deep fried you will have tortilla chips. Masa harina can also be used to make tamales, although there is a special kind for tamales (para tamales) that has a more course grind than the kind for tortillas.
The lime water used to treat the maize adds calcium and releases niacin from the niacytin which greatly increases the nutritional value of the corn.