Hydraulic Pressure Regulator Valve

hydraulic pressure
The force per unit area exerted in all parts of a hydraulic system by a liquid.
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties.
Pressure that exists in a system containing non-compressible liquid.

regulator valve
The main steam control of a locomotive. Various types of valve and various locations for it were to be seen during the history of the locomotive but it was normally at the top of the boiler where the steam was hottest and usually in a dome.