When the whole surface of a body's is acted upon by the forces, acting perpendicularly everywhere on it, the force per unit area is called Pressure. (see below fig)
Stress is also a force per unit area but it can be different on different surfaces. Also it is not necessary that the force should be perpendicular to the surface. For example, there is a stress on the cross section of a bar,( shown in below fig) but there is no stress on its sides.
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