Poisson’s ratio is a really important material property that helps describe how a material will deform under loading. It defines how much a material will deform in the lateral direction when a load is applied to it in the longitudinal direction.

This is expressed in terms of lateral and longitudinal strains in the equation below. $\nu$ is Poisson’s ratio.

The animated video below goes into more detail about Poisson’s ratio and explores some interesting topics like auxetic materials, which are materials that have a negative Poisson’s ratio, and incompressible materials, which are materials with a Poisson’s ratio of 0.5.