What is the meaning of valve?

noun

• A structure in a hollow organ (like the heart) with a flap to insure one-way flow of fluid through it.
• Device in a brass wind instrument for varying the length of the air column to alter the pitch of a tone.
• Control consisting of a mechanical device for controlling the flow of a fluid.
• The entire one-piece shell of a snail and certain other molluscs.
• One of the paired hinged shells of certain molluscs and of brachiopods. .