 The stress–energy tensor is defined as the tensor "T""αβ" of order two that gives the flux of the "α"th component of the momentum vector across a surface with constant "x""β" coordinate.In the theory of relativity, this momentum vector is taken as the four-momentum.In general relativity, the stress–energy tensor is symmetric,

In some alternative theories like Einstein–Cartan theory, the stress–energy tensor may not be perfectly symmetric because of a nonzero spin tensor, which geometrically corresponds to a nonzero torsion tensor.