What is pressure?

Pressure, symbolized as P or p in physics, measures the amount of continuous perpendicular force applied to a surface. Pressure is calculated by dividing the amount of force (typically measured in newtons) by the surface area that the force is being applied to. The SI unit for pressure is the pascal, which is the equivalent of one newton of square force per one square meter of surface area. However, pressure is often measured in non-SI units when calculating liquid or atmospheric pressure. An example of a common non-SI unit of pressure is the bar, a unit equivalent to 100,000 pascals that is most often utilized to measure the Earth's atmospheric pressure. Pressure is also a unit of ideal gas law, varying linearly with absolute temperature and the amount of substance, and inversely with the volume of the gas.
http://hyperphysics.phy-astr.gsu.edu/hbase/press.html