Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stressing force.Tensile deformat is considered positive and compressive deformation is considered to be -ve. The definition if Poisson's ratio contain a minus sign so that normal material have positive ratio. Poisson's ratio is also called Poisson coefficient .
n = - ∈trans / ∈longitudinal
where n = Poisson's ratio ∈t = transverse strain ∈l = longitudinal or axial strain