The negative ratio of lateral to axial strain in a stretched material.
About 0.3 for most metals, 0.35 to 0.4 for plastics, and approaching 0.5 for rubber. It describes why a bar gets thinner when pulled and why materials bulge when compressed.
It appears wherever a stress state has more than one axis — it is what converts a uniaxial tensile test into the shear modulus through G = E ÷ 2(1 + ν).
Ignoring it in a biaxial stress analysis. Assuming zero lateral contraction overestimates the stiffness of a constrained part.
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