Check valve

this siamese clappered inlet allows one or two inputs into a deluge gun

A check valve is a mechanical device, a valve, that normally only allows fluid to flow through it in one direction. A double check valve is often used as a backflow prevention device to keep potentially contaminated water from siphoning back into municipal water supply lines. A clapper valve is a type of check valve used in or with firefighting, and has a hinged gate (often with a spring urging it shut) that will only remain open in the outflowing direction.

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.

Nikola Tesla invented a deceptively simple one-way valve for fluids in 1920 (U.S. patent # 1,329,559).

In electronics, a diode functions in the same manner.