When a material is compressed, Poisson's ratio is the ratio of orthogonal expansion (measured as strain) to axial compression (also strain).
Most materials have Poisson's Ratios between 0.1 and 0.5, but negative ratios are possible. A material with a positive ratio will expand sideways when compressed; a material with a negative ratio will contract.
Determination of Poisson's ratio is often covered by general tensile testing standards, such ASTM D638 and ISO 527. A few testing standards are published separately, however. These include ASTM C469 for concrete in compression, ASTM D6790 for honeycomb cores, and ASTM E132 as a general method.
For an isotropic, homogeneous material, Young's (E), Bulk (K) and Shear (G) moduli are interconvertible with the help of Poisson's ratio (v):
E = 3K(1 - 2v) = 2G(1 + v)
A Poisson's ratio of 0.5 indicates that a material experiences zero volume change upon compression or elongation.
This is an important design consideration for rubbers, which have Poisson's ratios near 0.5, and are often used in applications that expect signification deformation. An otherwise flexible rubber part that is not given room to expand sideways when compressed will act rigid.