What is "pilot valve"

Pilot valve \Pilot valve\ (Hydraulics) A small hand-operated valve to admit liquid to operate a valve difficult to turn by hand.

A pilot valve is a small valve that controls a limited-flow control feed to a separate piloted valve. Typically, this valve controls a high pressure or high flow feed. Pilot valves are useful because they allow a small and easily operated feed to control a much higher pressure or higher flow feed, which would otherwise require a much larger force to operate; indeed, this is even useful when a solenoid is used to operate the valve.

Pilot valves are often used in critical applications (i.e., emergency and safety controls) and are human-operated. They can be set up as a push-to-activate or dead man's switch.