In statistics, a property displayed by some sequences of adjacent items not being independent of each other.

Autocorrelation is a mathematical tool used frequently in signal processing for analyzing functions or series of values; such as, time domain signals. It is the cross-correlation of a signal with itself.

Autocorrelation is useful for finding repeating patterns in a signal; such as, determining the presence of a periodic signal that has been buried under noise, or identifying the fundamental frequency of a signal which doesn't actually contain that frequency component, but implies it with many harmonic frequencies.