A check valve, clack valve, non-return valve, reflux valve, retention valve or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.

Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, or inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.

An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.

Relative Technical Terminology:
Cracking pressure — the inlet pressure at which the first indication of flow occurs (steady stream of bubbles). Cracking pressure is also known as unseating head (pressure).
Reseal pressure — the pressure at which there is no indication of flow. Reseal pressure is also known as sealing pressure or seating head (pressure).
Back pressure — the differential pressure between the inlet and outlet pressures

Applications:

Pumps:
Check valves are also used in the pumps that supply water to water slides. The water to the slide flows through a pipe which doubles as the tower holding the steps to the slide. When the facility with the slide closes for the night, the check valve stops the flow of water through the pipe; when the facility reopens for the next day, the valve is opened and the flow restarts, making the slide ready for use again. Check valves are often used with some types of pumps. Piston-driven and diaphragm pumps such as metering pumps and pumps for chromatography commonly use inlet and outlet ball check valves. These valves often look like small cylinders attached to the pump head on the inlet and outlet lines. Many similar pump-like mechanisms for moving volumes of fluids around use check valves such as ball check valves. The feed pumps or injectors which supply water to steam boilers are fitted with check valves to prevent back-flow.

Industrial processes:
Check valves are used in many fluid systems such as those in chemical and power plants, and in many other industrial processes.
Typical applications in the nuclear industry are feed water control systems, dump lines, make-up water, miscellaneous process systems, N2 systems, and monitoring and sampling systems. In aircraft and aerospace, check valves are used where high vibration, large temperature extremes and corrosive fluids are present. For example, spacecraft and launch vehicle propulsion propellant control for reaction control systems (RCS) and Attitude Control Systems (ACS) and aircraft hydraulic systems.

Check valves are also often used when multiple gases are mixed into one gas stream. A check valve is installed on each of the individual gas streams to prevent mixing of the gases in the original source. For example, if a fuel and an oxidizer are to be
mixed, then check valves will normally be used on both the fuel and oxidizer sources to ensure that the original gas cylinders remain pure and therefore nonflammable.
In 2010, NASA's Jet Propulsion Laboratory slightly modified a simple check valve design with the intention to store liquid samples indicative to life on Mars in separate reservoirs of the device without fear of cross contamination.

Domestic use:

Some types of irrigation sprinklers and drip irrigation emitters have small check valves built into them to keep the lines from draining when the system is shut off.
Check valves used in domestic heating systems to prevent vertical convection, especially in combination with solar thermal installations, also are called gravity brakes.
Rainwater harvesting systems that are plumbed into the main water supply of a utility provider may be required to have one or more check valves fitted to prevent contamination of the primary supply by rainwater.
Hydraulic jacks use ball check valves to build pressure on the lifting side of the jack.
Check valves are commonly used in inflatables, such as toys, mattresses and boats. This allows the object to be inflated without continuous or uninterrupted air pressure.