As far as I can tell, stress and traction seem to refer to similar ideas. According to Chapter 8 of Twiss and Moore's Structural Geology, traction is
Force per unit area on a surface of a specified orientation (Twiss, 129)
while stress is
A pair of equal and opposite tractions acting across a surface of specified orientation (Twiss, 129)
I don't understand this latter concept. Why have a term at all if the forces are equal but opposite? Then they should cancel out and be irrelevant right?
What is stress and how is it different from traction?