Pressure is a measure of the force exerted per unit area on the boundaries of a substance (orsystem). It is caused by the collisions of the molecules of the substance with the boundaries ofthe system. As molecules hit the walls, they exert forces that try to push the walls outward. Theforces resulting from all of these collisions cause the pressure exerted by a system on itssurroundings. Pressure is frequently measured in units of lbf/in2 (psi).