**1. n. [Geophysics]**

An elastic constant that is a measure of the compressibility of material perpendicular to applied stress, or the ratio of latitudinal to longitudinal strain. This elastic constant is named for Simeon Poisson (1781 to 1840), a French mathematician. Poisson's ratio (σ) can be expressed in terms of properties that can be measured in the field, including velocities of P-waves (*V _{P}*) and S-waves (

σ = ½ (*V _{P}*

Note that if *V _{S}* = 0, then Poisson's ratio equals 0.5, indicating either a fluid, because shear waves do not pass through fluids, or a material that maintains constant volume regardless of stress, also known as an ideal incompressible material. Poisson's ratio for carbonate rocks is ~0.3, for sandstones ~0.2, and greater than 0.3 for shale. The Poisson's ratio of coal is ~0.4.

**See:**
elastic constants,
P-wave,
S-wave,
velocity