You may wish to review how it appears in the governing equations for elastic deformation in sub-subunit 3.2.1. Poisson's ratio is commonly represented with the symbol nu.
Attribution:
Stephanie Redfern, Ranjeet (Ron) Agarwala, and Dr. Steve Gibbs. Mechanics I. The Saylor Academy 2014, http://www.saylor.org/courses/me102/