Poissons ratio is a measure of the Poisson effect, which is the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. It is the negative of the ratio of transverse strain to axial strain, which means it is the amount of transversal elongation divided by the amount of axial compression. For small values of these changes, the ratio is the amount of transversal expansion to the amount of axial compression. Poissons ratio is named after the French mathematician and physicist Siméon Poisson.

Most materials have Poissons ratio values ranging between 0.0 and 0.5. For soft materials, such as rubber, where the bulk modulus is much higher than the shear modulus, Poissons ratio is near 0.5. For open-cell polymer foams, Poissons ratio is near zero, since the cells tend to collapse in compression. Many typical solids have Poissons ratios in the range of 0.2–0.3.

In mechanics, Poissons ratio is the ratio of transverse contraction (or expansion) strain to longitudinal extension strain in the direction of stretching force. It is denoted by the Greek letter nu. For tensile deformation, Poissons ratio is positive, and for compressive deformation, it is negative. Poissons ratio for a material remains approximately constant within elastic limits.