A batted ball that bounces in fair territory and leaves play, most often over a fence or other boundary outside of the playing field. The batter who struck the ball and any runners on base at the time of the hit are awarded two bases.
The automatic double is often used as a colloquial substitute for the term “ground rule double,” but an automatic double is not always a ground rule double. A ground rule double is subject to the “ground rules” of the ballpark in which the game is being played—hence the name. For example, if a ball gets stuck in the ivy at Wrigley field, or is wedged under a bench in the bullpen, that is a ground rule double. As such, ground rule doubles are sometimes subject to interpretation by the umpiring crew. An automatic double, on the other hand, specifically refers to a ball bouncing fair and then leaving the field of play.