An abridging of mathematical notation that is particularly convenient when dealing with tensors (and so also applies to vectors and matrices). The convention is that if an index is repeated, then a sum over that index is implied. For example, the matrix multiplication C = AB would be written $c_j^i=a_k^i{b}_j^k$ with the summation over k suppressed and with the superscripts and subscripts denoting that a matrix is a (1,1)-tensor.