A savory crepe made of rice and urad dal (black gram, with skins removed) instead of wheat flour, common in South Indian cuisine. Served for breakfast or a snack, the rice and lentil batter is left to ferment overnight, and ladled onto a hot griddle greased with oil (or ghee). Typically, it's only cooked on one side (the crepe is thin enough that it cooks through) and served folded or rolled over (often with a filling inside, such as spiced potatoes (masala dosa), depending on regional tastes).
In Tamil Nadu, "dosai" (தோசை) are typically served with sambar and coconut chutney.
For a simplified recipe, see sneff's Rice and lentil pancakes with feta and balsamic grapes.
Sources:
Wikipedia contributors, "Dosa," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Dosa&oldid=259066135 (accessed December 29, 2008).