A frittata is a thick Italian type of omelette, that some food historians believe may pre-date the classic omelette. It differs from the classic French omelette in that it is cooked together with the chosen ingredients and served open-faced instead of  being folded over with the ingredients inside it as a filling. 

Traditionally it is partially cooked on the stove and finished off by broiling under a hot grill or baking in an oven, until just set. The ingredients that go into it may vary from a simple combination of onions, parmesan cheese and a little tomato sauce, as has been cooked and eaten by Italian peasants since time immemorial,  to one that is studded with meat, vegetables, cheese and even leftover pasta, especially in Naples. In Italy it is not uncommon to find it served cold, cut into large wedges.
It makes a substantial lunch or snack and excellent picnic fare. There are no specific ingredients that are meant to go into a frittata as the whole point or idea behind it is to use up leftovers in your kitchen.