When pulled in tension (Z), a sample gets longer and thinner, that is a contraction in the width (X) and breadth (Y). If compressed gets fatter.
Materials subject to tension shrink laterally. Those subject to compression, bulge. The ratio of lateral and axial strains is called the Poisson's ratio ν.
ν is dimensionless, sign shows that lateral strain is in opposite sense to longitudinal strain
Theoretical value for isotropic material: 0.25
Maximum value: 0.50, Typical value: 0.24 - 0.30