P-Wing is an item that first appeared in Super Mario Bros. 3.
This item can be obtained from a treasure chest in Toad's House or from Princess Peach's letters after defeating one of the Koopalings. It grants you unlimited flight for one level as long as the player isn't hit. Once the level ends, you will still retain the powers of a basic Super Leaf.
In Super Mario 3D Land, you can find a P-Wing inside a gold block after losing 10 lives in a level on Normal Mode. Using it will take you immediately to the end of the stage.