Positive Definite Function

A Positive definite Function $f$ on a Group $G$ is a Function for which the Matrix $\{f(x_i{x_j}^{-1})\}$ is always Positive Semidefinite Hermitian.

References

Knapp, A. W. ``Group Representations and Harmonic Analysis, Part II.'' Not. Amer. Math. Soc. 43, 537-549, 1996.