The cross-correlation $c_{xy}(\tau)$ of two functions $x(t)$ and $y(t)$ is the time-averaged product of the two functions with one delayed by time $\tau$, i. e.

$$c_{xy}(\tau) \equiv \langle x(t) y(t-\tau) \rangle$$

where $\langle \rangle$ represents time-average. This is also called the cross-power product.