1. . 
A mechanical device that permits fluid to flow or pressure to act in one direction only. Check valves are used in a variety of oil and gas industry applications as control or safety devices. Check valve designs are tailored to specific fluid types and operating conditions. Some designs are less tolerant of debris, while others may obstruct the bore of the conduit or tubing in which the check valve is fitted.
2. . 
A one-directional valve that is opened by the fluid flow in one direction and closes automatically when the flow stops or reverses direction.