What is Poisson's Ratio and what are typical values for common materials?

Poisson's Ratio (ν) is the ratio of lateral strain to longitudinal strain when a material is subjected to axial loading. It describes how much a material contracts in perpendicular directions when stretched. Typical values are 0.3 for steel, 0.33 for aluminum, 0.5 for rubber (incompressible), and 0.2-0.3 for concrete.