A term in baseball which refers to a rule in which all offensive players are awarded two bases after a ball, after being hit fair, goes out of play. There is no restriction on the number of runners that may advance; if the bases are loaded, all runners would advance, thus scoring two runs. Typically, this occurs after a ball bounces from the field into the stands. There are some other events which call for two bases to be awarded, such as a ball getting stuck in bushes or being deflected by a fan or other object on the field.
Prior to the existence of the ground-rule double, any ball hit fair, which subsequently made it over the fence, counted as home-run. This was changed in 1930 by the American League. It is called a "ground-rule" double because the ground rules at every stadium are different. There are exceptions and additions to the rules, such as what intrusive parts of the field may count as fair territory (such as rafters or lighting). One interesting part of the ground-rules of every field is that the umpire is considered part of the field. If a ball hits an umpire standing in fair territory, and then leaves the field of play, a ground-rule double is enforced.