What is the origin of the word tart?

A tart is a pastry dish, usually sweet, that is a type of pie with an open top not covered with pastry. Sweetened fruit is arranged on pastry, often puff pastry, with or without custard, after which the tart is baked in an oven. The Tarte Tatin is a particular kind of "upside-down" tart, of apples, other fruit, or onions. Savoury varieties include German Zwiebelkuchen (Onion tart), or Swiss cheese tart made from Gruyere.