What is the ratio of unit lateral deformation to unit longitudinal deformation called?

The ratio of unit lateral deformation to unit longitudinal deformation is known as Poisson's ratio. This ratio describes how a material deforms in the lateral direction when subjected to axial stress.

When a material is stretched or compressed, it not only changes in length but also in cross-sectional dimensions. Poisson's ratio quantifies this behavior by comparing the strain experienced in the direction of the applied load (longitudinal strain) to the strain experienced perpendicular to that load (lateral strain). A high Poisson's ratio indicates that the material tends to contract more in the lateral direction when stretched than a material with a low Poisson's ratio. This concept is vital in materials science and mechanical engineering, as it helps in understanding the mechanical behavior and properties of materials under different loading conditions.