Stress is the force per unit area on a body that tends to cause it to change shape.[2]
Stress is a measure of the internal forces in a body between its particles.[2] These internal forces are a reaction to the external forces applied on the body that cause it to separate, compress or slide.[2] External forces are either surface forces or body forces. Stress is the average force per unit area that a particle of a body exerts on an adjacent particle, across an imaginary surface that separates them.