Correlation or Cross-Correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them.

Correlation is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long-duration signal for a shorter, known feature. It also has applications in pattern recognition, single particle analysis, electron tomographic averaging, cryptanalysis, and neurophysiology. Another variant of correlation could be auto-corelation.

Source: Wikipedia.