Autocorrelation's the cross-correlation of a signal with itself. It's a mathematical tool for finding repeating patterns, such as the presence of a periodic signal which has been buried under noise, or identifying the missing fundamental frequency in a signal implied by its harmonic frequencies. It's used frequently in signal processing for analyzing functions or series of values, such as time domain signals. Informally, it's the similarity between observations as a function of the time separati… (More on Autocorrelation)