A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid to flow through it in only one direction.
Tilting-disc inconel check valve
Vertical lift check valve
A disassembled poppet check valve
Large butterfly check valve for a water pump
A valve is a device or natural object that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
These water valves are operated by rotary handles.
Valve disc
The shut off butterfly valve for a Francis turbine at Gordon Power Station, Tasmania
Ball valve