In baseball, a tag out, sometimes just called a tag, is a play in which a baserunner is out because a fielder touches him with the ball or with the hand or glove holding the ball, while the ball is live and the runner is in jeopardy of being put out - usually when he is not touching a base.
A baserunner is in jeopardy when:
A tag is therefore the most common way to retire baserunners who are not in danger of being forced out, though a forced runner may be tagged out in lieu of stepping on the forced base. Additionally, a tag out can be used on an appeal play.
Runners attempting to advance are sometimes thrown out, which means that a fielder throws the ball to someone covering the base, who then tags the runner before the runner touches the base. A runner who leads off a base too far might be picked off; that is, the pitcher throws to a fielder covering the base, who then tags the runner out.
When a runner is tagged out, a farther advanced runner who had been forced to advance no longer has to do so. For example, when a sharply hit ball is caught on one hop by the first baseman, he might immediately tag out the runner at first who is forced to advance to second; but when this is done a runner already at second is no longer forced to advance to third base. The result of such a tag is called "removing the force".
If a defensive player tags the runner with his glove and the baseball is in his other hand, or with his free hand while the baseball is in his glove, then the runner is not out. Attempted tag outs sometimes can instigate fights in baseball. Often the infielder will tag the baserunner too hard, or the baserunner will slide into base with enough speed and force to knock over the infielder attempting to tag him out. Additionally, the runner may run the catcher over in an attempt to make him drop the ball. However, after rule changes put into effect in the 2010s to protect the catcher, if the runner is seen as using excessive force by lowering their shoulders or using their elbows, the baserunner will be called out. Also, runners must slide and avoid when possible, or they could be ruled out.