The Designated runner, also called the automatic runner or ghost runner, is the baserunner who begins an extra inning on second base when the tiebreaker rule is in effect. The term came into common usage in 2020 when Major League Baseball adopted the rule as part of measures to limit the number of innings played during the shortened season caused by the Coronavirus pandemic.
The term is copied from "Designated hitter", but there are major differences between the two. Who is the designated runner is a function of who was the final batter in the previous inning, and not a choice of the manager. And if the game goes on for a number of extra innings, many different players in the line-up can occupy the role at one time or another in the same game. In addition, while the designated hitter is selected for his hitting ability, the designated runner assumes the role randomly, and may not in fact be a good baserunner. There is always an option to replace that player with a pinch-runner, but this will normally have implications on the team's defensive alignment in the bottom of the inning, if that is the case, or in any future inning.
- Will Leitch: "5 reasons to the love automatic-runner rule", mlb.com, May 13, 2021.