TensorContract

TensorContract[tensor,{{s11,s12},{s21,s22},…}] yields the contraction of tensor in the pairs {si1,si2} of slots.

Examples

Contract a matrix (compute trace):

TensorContract[{{a, b}, {c, d}}, {{1, 2}}]
(* a + d *)

Please visit the official Wolfram Language Reference for more details.