Stress
Stress occurs in any material which supports a load. Stress is the way a material transfers the load through its molecules. It's like an internal pressure resisting the external applied loads.
Stress, like pressure, is force per unit area, ie 
Remember that force is measured in units of newtons (N) or kilonewtons (kN) and the unit of area is square millimetres (mm2) or square metres (m2).
Stress is measured in pascals (Pa), kilopascals (kPa) or megapascals (MPa). The most common unit used for structural materials is MPa as the other units are too small. For example, the concrete strength for a typical reinforced concrete slab is 25 MPa.