Discovered by Francis Dalton in his report “Regression towards Mediocrity in Hereditary Stature”1. He called it Regression towards Mediocrity.
Galton observed that extreme characteristics (e.g., height) in parents are not passed on completely to their offspring. Rather, the characteristics in the offspring regress towards a mediocre point (a point which has since been identified as the mean).2
Flip a coin 2 times. If you get tails both times, is the chance of getting tails $100%$? No, you just haven’t flipped the coin enough times. The true probability is still $50%$, and if you continue to flip, your estimated probability will eventually regress towards that.