[L. valva, the leaf of a folding door.] A kind of flap or lid in a pipe or a blood-vessel which allows a fluid or gas to flow only in one direction, as in the common pump. There are a flap-valve, puppet-valve, ball-valve and slide-valve. A safety-valve is held shut by a spring or weight, and opens automatically to allow steam, gas, or water to escape when the pressure becomes too great. The valves in the blood-vessels assist the flow of blood through the veins.