The operation of a hydraulic device can be explained in terms of?

The principal is called Pascal's Principle.

Pascal's principle states that a pressure applied to an enclosed fluid is transmitted everywhere in the fluid. Hence, if a pressure is applied to one side of an enclosed fluid, all the other walls containing the fluid feel the same pressure. The pressure is transmitted without being diminished.

Fluid generally refers to either a liquid or a gas. If a pressure is applied to a compressible gas, Pascal's principle still applies, but the volume of the gas will change. For Pascal's principle to be useful to hydraulics, the fluid should be an incompressible liquid, which will transmit the applied pressure without changing its volume.