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 σ  is defined as :σ=lateral  strainlongitudinal  strain=−ΔrrlΔlThe theoretical value of σ lies between -1 to 0.5 & practical value of σ lies between  - zero to 0.5.