Pressure is stress that is exerted uniformly (or the same way) in all directions.
Pressure is measured in units of force applied per unit of area.
Force exerted on a square inch of area of a confined liquid is transmitted at every angle to every square inch of area of the interior of the vessel. If one of the areas of the interior of the vessel is a second piston, the force would be transmitted to every square inch of its area as well.
In a schematic drawing (shown on the right) of a vessel which contains a liquid. Piston A is exerting a force of 2000 lbs. on the confined liquid. The effective area of Piston A is 1 Sq. In., so the pressure exerted on the liquid by Piston A is 2000 psi.
Hydraulic / Mechanical example:
If pump piston A is pushed down 12 inches, then cylinder piston B will rise 1 inch.
Pressure = 2,000 PSI
Piston A @ 1 sq. in. = 2,000 lbs.
Piston B @ 12 sq. in. = 24,000 lbs.