When a bar of a solid material is loaded in tension longitudinally, the longitudinal extension is accompanied by a lateral contraction. The ratio between the lateral contraction per unit length and the longitudinal extension also per unit length is known as the Poisson's ratio. In the elastic regime, this ratio is a material constant with a value of about 0.3 for most metals.

In the literature the Greek letter ν is commonly used as the symbol for the Poisson's ratio.

Consider an elemental cube with its sides parallel to the three coordinate axes x,y,z. Under a stress system σ_{x}, σ_{y}, σ_{z} the strains are ε_{x}, ε_{y}, ε_{z}. The relationship between strains and stresses in the elastic regime is given by *Hooke’s laws,*

where E is *Young’s modulus* and ν is the Poisson's ratio.