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:

1. 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);
2. he is touching a base he has been forced to vacate because the batter became a baserunner;
3. he has not tagged up on a caught fly ball;
4. he failed to touch a previous base, or to touch them in order; or
5. 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.