An NRV valve, or Non-Return Valve (also commonly known as a check valve, one-way valve, or clack valve), is a mechanical device that allows fluid (liquid or gas) to flow in only one direction while automatically preventing backflow in the opposite direction.

The valve operates passively without external control:
1. Forward flow pressure pushes an internal mechanism (like a disc, ball, or flap) open.
2. Reverse flow or back pressure forces it closed, often aided by gravity, a spring, or the fluid itself.