The absolute value of the ratio of the transverse strain to the corresponding axial strain, in a body subjected to uniaxial stress; usually applied to elastic conditions. If a square bar is stressed in a testing machine in the direction of its length so that the length increases, there is a contraction in each opposite direction, which produces a decrease in the thickness of the bar. The ratio between the contraction at right angles to a stress and the direct extension is called the Poisson's ratio. Its value in steel is in the order of 0.28.