Usually this refers to the autocorrelation coefficient. Consider any 1D signal with periodicity $\pi$. Now let's look at the autocorrelation integral: > $\int_{-\infty}^{\infty}\! f(t)f(t-\tau)\,\mathrm{d}x$ For varying $\tau$, the autocorrelation will have a maximum for $\tau$ equalling $\pi$ and its multiples. Thus autocorrelation can be used to study the periodicity of a signal. This is often sort of colloquially used to indicate that certain parts of a signal are very similar or even identical. The analogue for two different signals would be the cross correlation. It can be used to study the similarity of two separate signals.