Flap

1. A flat, usually thin piece attached at only one side. The tent flap helps keep the tent dry.
2. A projecting or hanging piece usually intended to double over and protect or cover. The flap of an envelope.
3. The act of waving or fluttering. You could hear the flap of a flag in the wind.
4. A blow given with something flat; a slap. The flap sent him reeling.
5. A variable control surface on the trailing edge of an aircraft wing, used primarily to increase lift or drag. You lower the flaps when landing.
6. Either end of the folded ends of a book jacket that fits inside the front and back covers. The back flap has a photo of the author.

7. Tissue that has been partially detached and used in surgical grafting to fill an adjacent defect or cover the cut end of a bone after amputation. Amputations work better if you have a flap to soften the end.
8. A commotion or disturbance; There was a flap in Congress over the defense budget.
9. To wave "the arms, for example) up and down, as if with wings to try to fly. Big heavy birds flap the wings furiously to lift off of the water.