Matrix coefficient - Wikipedia
https://en.wikipedia.org/wiki/Matrix_coefficientIn mathematics, a matrix coefficient (or matrix element) is a function on a group of a special form, which depends on a linear representation of the group and additional data. Precisely, it is a function on a compact topological group G obtained by composing a representation of G on a vector space V with a linear map from the endomorphisms of V into V 's underlying field. It is also called a representative function. They arise naturally from finite-dimensional representations of G as the m…