Definition 1 of 2
Definition
A one-way flow-control device installed in a fluid or gas line that permits flow in one direction and blocks it in the opposite direction. Common designs use a spring-loaded ball, flapper, or poppet that lifts off its seat when pressure builds on the inlet side and reseats when pressure reverses or equalizes.
Plain English
A valve that lets fluid or air move through it one way only. If anything tries to flow back the other way, the valve closes and stops it.
Context Anchor
Seen in aircraft fuel, oil, hydraulic, vacuum, and pneumatic system descriptions.
Derivation
From 'check' meaning to stop or restrain (as in 'hold in check'), and 'valve' from Latin valva, meaning the leaf of a folding door. So literally: a door-like device that stops something. That fits exactly -- it stops reverse flow.
Why Pilots Care
Ensures proper system pressure and prevents backflow that could cause fuel starvation or hydraulic failure.
Analogy
A check valve works like a one-way door in a hallway. People can push through from one side, but the door will not let them come back the other way.
Intuition Check
Check does not mean inspect here. In check valve, check means stop or prevent reverse flow.
Example Sentence 1
A check valve in the fuel crossfeed line prevents fuel from flowing backward into the wrong tank.
Example Sentence 2
Hydraulic check valves maintain pressure in the landing gear system after the pump shuts off.