Poisson’s Ratio:

When an object is elongated longitudinally in the direction of the tensile force, then there will be a lateral contraction. this means that the contraction is in a direction perpendicular to the direction of tensile force. The ratio of lateral strain to longitudinal strain within the elastic limit is called Poisson’s Ratio $(\sigma)$.

Let us consider a wire of length $L$ and diameter $D$, undergoes an eleongation $l$ along the direction of applied tensile force $F$, and also undergoes a contraction of amount $d$ along the direction perpendiculer to the direction of tensile force $F$. Then Poisson’s Ratio wil be

Is Poisson’s Ratio an Elastic Modulus?

Poisson’s ratio is not an elastic modulus as it is the ratio of two types of strains, not the ratio of stress to strain. It is a dimensionless pure number.