Pressure (symbol: p) is defined as force per unit area.The pressure is measured in pascals (SI unit) or in dynes (cgs unit).Pascal is equivalent to one newton per square meter(adopted for the SI unit by the 14th CGPM in 1971).
The unit is named after Blaise Pascal, the eminent french physicist and mathematician, noted for his experiments with a barometer, an instrument to measure air pressure.
Is defined by:
P=F/A
p is the pressure
F is the normal force
A is the area.
For an object sitting on a surface, the force pressing on the surface is the weight of the object, but in other situations it might have a different area in contact with the surface and therefore exert a different pressure.
Energy density per unit volume has the same physical units as pressure and in a fluid pressure may be considered to be a measure of energy per unit volume or energy density.
For a force exerted on a fluid, we can define:
P= FORCE / AREA= ENERGY / VOLUME
When you calculate pressure of a liquid at rest, the medium is treated as a continuous distribution of matter, but when you deal with a gas pressure, it must be approached as an average pressure from molecular collisions with the walls.
For gases, pressure is sometimes measured not as an absolute pressure (is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum) but relative to atmospheric pressure;Measurements like this are called gauge pressure.Gauge pressure is sometimes also called overpressure.