The idea that stock prices revert to a long term level. Hence, if there is a shock in prices (unexpected jump, either up or down), prices will return or revert eventually to the level before the shock. The time it takes to revert is often referred to as the time to reversion. If the process is very persistent, it might take a long time to revert to the mean. The key difference between a mean-reverting process and a random-walk is that after the shock, the random-walk price process does not return to the old level.