When some external system of forces or loads act on a body, the internal forces (equal and opposite) are set up at various sections of the body, which resist the external forces. This internal force per unit area at any section of the body is known as unit stress or simply a stress. It is denoted by a Greek letter sigma (σ). Mathematically,
Stress, σ = P/A
P = Force or load acting on a body, and
A = Cross-sectional area of the body.
In S.I. units, the stress is usually expressed in Pascal (Pa) such that 1 Pa = 1 N/m2. In actual practice, we use bigger units of stress i.e. megapascal (MPa) and gigapascal (GPa), such that
1 MPa = 1 × 106 N/m2 = 1 N/mm2
and 1 GPa = 1 × 109 N/m2 = 1 kN/mm2
A textbook of Machine Design by R.S.Khurmi and J.K.Gupta