Meaning of Poisson's ratio

Meaning of Poisson's ratio

Definition of Poisson's ratio
Poisson's ratio n is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative. The definition of Poisson's ratio contains a minus sign so that normal materials have a positive ratio.
n = - etrans / elongitudinal

Strain e is defined in elementary form as the change in length divided by the original length.
e = DL/L.