Stress is defined as the strength of a material per unit area or unit strength. It is the force on a member divided by area, which carries the force, formerly express in psi, now in N/mm^{2} or MPa.

where P is the applied normal load in Newton and A is the area in mm^{2}. The maximum stress in tension or compression occurs over a section normal to the load.

Normal stress is either tensile stress or compressive stress. Members subject to pure tension (or tensile force) is under tensile stress, while compression members (members subject to compressive force) are under compressive stress.

Compressive force will tend to shorten the member. Tension force on the other hand will tend to lengthen the member.