When a body undergoes a linear tensile strain it experience a lateral contraction also. The ratio of lateral contraction to longitudinal strain is known as

Poission’s ratio $\sigma$  is defined as :

$\sigma =\frac{lateralstrain}{longitudinalstrain}=-\left(\frac{\frac{\Delta r}{r}}{\frac{l}{\Delta l}}\right)$

The theoretical value of $\sigma$ lies between -1 to 0.5 & practical value of $\sigma$ lies between  - zero to 0.5.