Poisson's Ratio

Poisson's ratio (ν), is the ratio, when a sample object is stretched, of the contraction or transverse strain (perpendicular to the applied load), to the extension or axial strain (in the direction of the applied load).

When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect. Poisson's ratio ν (nu) is a measure of the Poisson effect

In the above picture, the stress is acting in X axis, but change in object is evident in Y and Z axes also. Poisson's effect is all about this change and Poisson's ratio is a measure of this effect and is given by 

For ideal material, the ratio is 0.5. But in general it ranges from 0.25 to 0.40.

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PS: Some of the info here is taken from Wikipedia. Please do consult them for more info