Stress

When an external force is applied on a body, internal resistance is developed within the body to balance the effect of externally applied forces. The resistive force per unit area is called as stress. 

In other words,

When a body is acted upon by some load or external forces, it undergoes deformation (i.e. change in shape or dimensions) which increases gradually. During deformation, the material of the body resists the tendency of the load to deform the body and when the load influences is taken over by the internal resistance of the material of the body, it becomes stable. This internal resistance which the body offers to meet with the load is called stress. 

σ=P/A

Stress can be considered either as total stress or unit stress. Stress is expressed in KN/m², N/mm²