Tortilla (Spanish, “little cake”) refers to two entirely different food items. In Spanish cuisine, a tortilla is an omelette-like dish of eggs and potatoes or other fillings; but in Mexico the word refers to the round, thin, unleavened flatbread made of corn (maize) that has been eaten in that country for centuries.
The Spanish tortilla, although it is an egg dish, bears less resemblance to the French omelette – a fluffy disk of scrambled eggs folded around a filling – than to the Persian kookoo, the Arab eggah, and the Italian frittata. The fillings, which can range from leafy green vegetables to potato slices to fish and cured meats, are mixed in with the scrambled eggs before being fried in a pan. The ratio of eggs to filling varies: the dish may consist of mostly eggs and a few vegetables, or, as in the potato tortilla that is ubiquitous in tapas restaurants, the filling may take the lead role with the eggs serving as a binder. Spanish tortillas may be served hot or at room temperature. 
The completely unrelated Mexican tortilla has been a staple from long before the European contact; since that time they have come to be made from wheat as well as from the traditional corn. Tortillas are eaten by themselves, as “bread,” or used as the basis for countless other dishes such as tacos, enchiladas, tostadas, chilaquiles, and (with wheat tortillas) burritos and chimichangas. 
Traditionally, Mexican tortillas are made of corn boiled with unslaked lime (that is, calcium hydroxide, not the citrus fruit), then ground into a dough called “masa,” flattened, and baked. 
- Alan Davidson, “Omelette,” The Oxford Companion to Food (Oxford: Oxford University Press, 1999); Penelope Casas, Tapas: The Little Dishes of Spain (New York: Knopf, 1986), 195-201.
- Alan Davidson, “Tortilla,” The Oxford Companion to Food; Elizabeth Lambert Ortiz, The Complete Book of Mexican Cooking (New York: M. Evans, 1967; Ballentine, 1985), 25ff.
- “Masa in the Limelight,” Art Culinaire, Spring 2006, online at http://findarticles.com/p/articles/mi_m0JAW/is_80/ai_n16346454