**Mean reversion** is a phenomenon where, if a data point is far from the historical average values, then the tendency of the next data point is, to be nearer to the average value. In other words, when a particular instance in a mean-reverting data series is an outlier, the next data point has lower probability of being above the outlier, but its highly probable that it would be contained between the outlier and the historical mean. Also, there exists a strong tendency in place, for this data series to gradually gravitate towards the historical mean. It is also known as regression to the mean.