Note that stress has units of pressure and that strain is dimensionless; the ratio has units of pressure. You may wish to refer to the figure in sub-subunit 2.2.2.
Attribution:
Stephanie Redfern, Ranjeet (Ron) Agarwala, and Dr. Steve Gibbs. Mechanics I. The Saylor Academy 2014, http://www.saylor.org/courses/me102/