Unit pressure or simply called pressure is the amount of force exerted by a fluid distributed uniformly over a unit area.
If the unit pressure is not uniform over the unit area, it can be expressed as the sum of differential pressure.
Blaise Pascal (1623 – 1662)
Since fluid at rest cannot resist shearing stress, pressure is always at right angle to the area where it is acting. It is also worthy to note that the total hydrostatic force F = pA, which can be found by cross multiplication.