What is "flap valve"

Flap \Flap\, n. [OE. flappe, flap, blow, bly-flap; cf. D. flap, and E. flap, v.] Anything broad and limber that hangs loose, or that is attached by one side or end and is easily moved; as, the flap of a garment.

A cartilaginous flap upon the opening of the larynx.
--Sir T. Browne.

2. A hinged leaf, as of a table or shutter.

3. The motion of anything broad and loose, or a stroke or sound made with it; as, the flap of a sail or of a wing.

4. pl. (Far.) A disease in the lips of horses.

5. (Aeronautics) a movable part of an airplane wing, used to increase lift or drag, especially when taking off or landing. used often in the plural.

Flap tile, a tile with a bent up portion, to turn a corner or catch a drip.

Flap valve (Mech.), a valve which opens and shuts upon one hinged side; a clack valve.