Measure how similar a signal is to a time-shifted version of itself
Pitch Detection: Find the fundamental frequency of speech or music by locating the first major peak after lag 0.
Periodicity Detection: Identify repeating patterns in time series data (machine vibrations, ECG, etc.).
Noise Reduction: Random noise has near-zero autocorrelation at non-zero lags, while periodic signals maintain correlation.
Wiener-Khinchin Theorem: The autocorrelation and power spectral density are Fourier transform pairs.