A door; especially, one of a pair of folding doors, or one of the leaves of such a door.
- Anatomy A membranous structure in a hollow organ or passage, as in an artery or vein, that folds or closes to prevent the return flow of the body fluid passing through it.
- Any of various devices that regulate the flow of gases, liquids, or loose materials through piping or through apertures by opening, closing, or obstructing ports or passageways.
- The movable control element of such a device.
- Music A device in a brass wind instrument that permits change in pitch by a rapid varying of the air column in a tube.
- Biology One of the paired, hinged shells of certain mollusks and of brachiopods.
- Biology One of the two silicified halves of the cell wall of a diatom.
- Biology The entire, one-piece shell of a snail and certain other mollusks.
- Botany One of the sections into which the wall of a seedpod or other dehiscent fruit splits.
- Botany A lidlike covering of an anther.
- Chiefly British An electron tube or a vacuum tube.
- Archaic Either half of a double or folding door.
- To provide with a valve.
- To control by means of a valve.