What vegetable is added to eggs to make a Spanish omelette?

Spanish omelette or Spanish tortilla is a traditional dish from Spain. Celebrated as a national dish by Spaniards, it is an essential part of Spanish cuisine. It is an omelette made with eggs and potatoes, optionally including onion. It is often served at room temperature as a tapa. It is commonly known in Spanish-speaking countries as tortilla de patatas, tortilla de papas, tortilla española. The first reference to the tortilla in Spanish is found in a Navarrese document, as an anonymous "Mousehole's memorial" addressed to the Navarra region's court in 1817. It explains the sparse conditions of Navarre's farmers in contrast with those in Pamplona (the capital) and la Ribera (in southern Navarre).

Source: Wikipedia