Stress tensor
From Glossary of Meteorology
stress tensor
It has nine components, one for each of the coordinate faces of an imaginary fluid element upon which the stress acts (j = x, y, z) and for each direction in which the stress is directed (i = x, y, z) By definition, an inviscid fluid is one in which the six tangential stresses (i ≠ j) are zero, and the three normal stresses (i =j) are equal to the negative of the pressure.
