It can also be defined as the thrust [compressive force acting perpendicularly to the surface of a body] acting per unit area.
It is proved that less the area of cross section the more is the pressure and vice versa.
To measure the amount of pressure people use these units:
Pounds per square inch - (psi, PSI, lb/in2, lb/sq in)
Commonly used in the U.S., but not elsewhere. Normal atmospheric pressure is 14.7 psi, which means that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds.
Atmosphere - atm
Normal atmospheric pressure is called 1 atmosphere. 1 atm = 14.6956 psi = 760 torr.
P=F/A Torr - torr
This is called the Torr because it is based on the original Torricelli barometer design. One atmosphere of pressure will push the column of mercury (Hg) in a mercury barometer to a height of 760 millimeters. A pressure that causes the Hg column to rise 1 millimeter is called a torr. 1 atm = 760 torr = 14.7 psi.
Bar - bar
The bar is almost the same as the atmosphere unit. One bar = 750.062 torr = 0.9869 atm = 100,000 Pa.
Millibar - mb or mbar
Pascal - Pa
pascal is the SI unit of pressure. It was named after the French physicist Blaise Pascal. 1 pascal = a force of 1 newton per square meter (1 newton = the force needed to accelerate 1 kilogram one meter per second per second = 1 kg.m/s2; this is actually quite logical for physicists and engineers, honest). 1 pascal = 10 dyne/cm2 = 0.01 mbar. 1 atm = 101,325 Pascals = 760 mm Hg = 760 torr = 14.7 psi.
Kilopascal - kPa
The prefix "kilo" means "1,000", so one kilopascal = 1,000 Pa. Therefore, 101.325 kPa = 1 atm = 760 torr and 100 kPa = 1 bar = 750 torr.
Megapascal - MPa
The prefix "mega" means "1,000,000", so one megapascal = 1,000 kPa = 1,000,000 Pa. Pressures in the megapascal range are common in high-pressure air compressors and hydraulic systems, where high pressure liquids are used to do useful work.
Gigapascal - GPa
The prefix "giga" means "1,000,000,000", so one gigapascal = 1,000 MPa = 1,000,000 kPa = 1,000,000,000 Pa = 9,870 atm = 10,000 bar. Pressures of several gigapascals can change graphite to diamond or make hydrogen a metallic conductor!