A tendency to return to a previous average. The term regression dates back to Francis Galton and his work concerning the heights of children in different generations. The heights of children of exceptionally tall (or short) parents "regress" to the mean of the population. So if you see a result that is far above the current level, such as a baseball player hitting 70 home runs in a season, you should forecast that the next result will not be so outstanding. (See also regression to the mean.)