Well, what is Poisson’s ratio? Denoted as ν, Poisson’s ratio is named after Simeon Poisson, which is a measure of the tendency of both the strains acting opposite to each other. It is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied load. As most common materials become thinner in cross section on being stretched, Poisson’s ratio has a positive value for them.