The simple linear correlation of a time series with its own past; that is, the correlation of the sequence of values x(t) with the sequence of values x(t + τ) occurring τ units of time later. The time displacement τ is called the lag. The autocorrelation function is the autocorrelation for variable lag. The autocorrelation coefficient is the product-moment correlation coefficient that relates the variables x(t) and x(t + τ). See serial correlation.