What is pressure?

Pressure, pronounciation:\ˈpre-shər\
Function: noun
Etymology: Middle English, from Anglo-French, from Late Latin pressura, from Latin, action of pressing, pressure, from pressus, past participle of premere
Definitions:
1 a: the burden of physical or mental distress b: the constraint of circumstance : the weight of social or economic imposition
2: the application of force to something by something else in direct contact with it : compression
3archaic : impression, stamp
4 a: the action of a force against an opposing force b: the force or thrust exerted over a surface divided by its area c: electromotive force
5: the stress or urgency of matters demanding attention : exigency
6: the force of selection that results from one or more agents and tends to reduce a population of organisms
7: the pressure exerted in every direction by the weight of the atmosphere
8: a sensation aroused by moderate compression of a body part or surface

(sources:http://www.merriam-webster.com/dictionary)