A check valve, clack valve, non-return 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.

Heart valves are essentially inlet and outlet check valves for the heart ventricles, 

since the ventricles act as pumps.