1. n. [Drilling]
A check valve that has a spring-loaded plate (or flapper) that may be pumped through, generally in the downhole direction, but closes if the fluid attempts to flow back through the drillstring to the surface. This reverse flow might be encountered either due to a U-tube effect when the bulk density of the mud in the annulus is higher than that inside the drillpipe, or a well control event.
