In baseball, a tag out, sometimes just called a tag, is a play in which a baserunner is out because he is touched by the fielder's hand holding a live ball while the runner is in jeopardy. A baserunner is in jeopardy when:
- he is not touching a base (other than after overrunning first base or when advancing to an awarded base, such as on a base on balls);
- he is touching a base he has been forced to vacate because the batter became a baserunner (a forced runner)
- he has not tagged up on a caught fly ball;
- he failed to touch a base when he last passed it, or to touch them in order; or
- he is touching a base that a preceding baserunner is also touching, except when forced to vacate the previous base because the batter became a baserunner
A tag is the most common way to retire baserunners who are not in danger of being forced out (as in (1) above), but a tag can put runners out on a force or a non-force play, as well as on an appeal play (as in (3) or (4) above). 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.
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 the tag out. These often lead to altercations that may culminate in a bench-clearing brawl. Such altercations are much less common in amateur baseball.