Principal stresses, differential stress কী?

The principal stresses at a point are the normal stresses $\sigma_1 \ge \sigma_3$ on the two planes free of shear stress, and their directions are the principal directions. In three dimensions there are three, $\sigma_1 \ge \sigma_2 \ge \sigma_3$, on three perpendicular planes. The differential stress is $\sigma_1 - \sigma_3$, and the mean stress in the plane is $(\sigma_1+\sigma_3)/2$.