(automotive) A small butterfly valve fitted to the intake manifold just before the cylinder head intake ports of many modern automotive diesel engines, and around which air can pass even when it is closed.

A pet door.

A doggie door.

A device that allows water to flow in one direction only through a culvert; it is used especially to drain surface water from coastal marshes at low tide

A flap that hangs down behind a vehicle's wheel so that mud and other things kicked up by the wheel will hit the flap instead of a vehicle behind.

(rare, chiefly in the plural, vulgar, slang) A woman's labium.

A hinge in the rotor assembly of a helicopter that allows each blade to have some vertical movement

An outfall fitted with a metal flap so that floodwater will not back up

(slang) The flap for defecation in a union suit.

(aviation) A small door on the bottom of an aircraft's cowling, used to direct additional airflow over the engine in order to improve cooling.