Poisson's ratio

In material science, Poisson's ratio is the ratio of two dimensionless numbers: the transverse to longitudinal strain.

When a metal bar under tension is elongated, its width is slightly diminished. This lateral shrinkage constitutes a transverse strain that can be expressed as the change in the width divided by the original width (hence strain is dimensionless). Similarly, the longitudinal strain is the change in length divided by the original length of the metal bar under lengthwise tension or compression. The ratio of the two strains is Poisson's ratio. This ratio is named for the French mathematician Siméon-Denis Poisson.