Flap

noun

2. A projecting or hanging piece usually attached to something on one side and often intended to protect or cover.

   "the flap of an envelope."

3. Either of the folded ends of a book jacket that fit inside the front and back covers.
4. A variable control surface on the trailing edge of an aircraft wing, used primarily to increase lift or drag.
5. A piece of 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.