The degree of agreement between two unlike signals.

a way of determining how similar two signals are using a weighted moving average

The mathematically derived measure of similarity between two functions or signals. Cross-correlation also refers to the process of determining this similarity, and is accomplished by multiplying the two signals together and integrating the result over time. If the result is zero, the two signals are said to be uncorrelated, or orthogonal.

an equation describing the complex conjugate of one complex function, convolved with a second complex function. More at mathworld.com.