External loads and irregular distributions of temperature give rise to mechanical stresses inside a body. If a unit force dF is applied on a unit area dA of an imaginary section, the resultant stress is defined by the quotient :

s _res = . Breaking down dF into a normal (dF_n) and a tangential component (dF_t) in relation to the element of area produces the normal stress s = (tension or pressure) and the shear stress t = .

Stresses are a measure of a body's loading. Their calculation is an exercise of theoretical mechanics.