Relief valve
A pressure-control valve that relieves any pressure over the amount for which it is set. They are damage-preventing units used in both hydraulic and pneumatic systems. In an aircraft hydraulic system, pressure relief valves prevent damaging high pressures that could be caused by a malfunctioning pressure regulator, or by thermal expansion of fluid trapped in portions of the system.
Aviation Maintenance Technician Handbook — Airframe, p. G-28
Taught in PP-G Procedures, PP-G Concepts, AF-F Concepts, AF-O Concepts, GEN-A Concepts, PP-G Quick reference.