A violation called when a defensive player touches the ball when the shot is on its way down or after it touches the rim or the backboard. When goaltending is called, the shooter is awarded the points. It can also be called on an offensive player when he touches the ball while it's still within the imaginary cylinder that extends above the rim. In that case, the points do not count.
It was said that offensive goaltending was instituted in 1958 because of former Boston Celtics center Bill Russell and his dominance near the basket while he was in the NCAA. While he was playing for the University of San Francisco, he often guided in shots with his hands above the rim.