Internal restoring force per unit area of the body on which it acts is stress. Deformation happens (change in shape or in size or both) when an external force is acting on an elastic body. There is equal and opposite force developed inside the body to bring the body back to its original shape. This force is known as the restoring force.
The formula for stress is given as,
Stress = Applied Force / Area
Stress = F /A
The unit for stress is newton metre-2 with symbol N m-2 or ‘pascal’ with symbol ‘Pa’.