In baseball, an out occurs when the defensive, or fielding, team effects any of a number of different events, and the umpire rules a batter or baserunner out. When a player is called out, he is said to be retired. When three outs are recorded in an inning during a team's turn at offense, that team's half of the inning finishes, and it is said that "the side is retired."
- The batter is out when:
- the third strike is pitched and caught in flight;
- on any third strike, if a baserunner is on first and there are fewer than two outs;
- he bunts foul with two strikes;
- he is hit by his own fair ball, outside of the batter's box, before the ball is played by a fielder;
- he commits interference;
- he fails to bat in his proper turn and is discovered in an appeal;
- he hits a pitch while one foot is entirely outside of the batter's box;
- he steps from one batter's box to the other when the pitcher is ready to pitch; or
- he is found to have used an altered bat.
- The batter-runner is out when:
- he fails to reach first base before a fielder with a live ball in his possession touches first base or tags the batter-runner (except when the batter is awarded first base, such as on a base on balls);
- a batted ball is caught in flight (fly out); or
- he hits an infield popup while the infield fly rule applies.
- Any baserunner, other than the batter-runner, is out when:
- he is forced out; that is, he fails to reach his force base before a fielder with a live ball touches that base;
- a fielder catches a batted ball in flight, and subsequently, some fielder with a live ball in possession touches the runner's time of pitch base before the runner tags up (appeal play);
- while he is attempting to reach home plate with fewer than two outs, the batter interferes with a fielder and such action hinders a potential tag out near home plate;
- he is found to have committed a mockery of the game, for example a stolen base of first from second;
- he is found to be an illegal substitute.
- Any baserunner, including the batter-runner, is out when:
- he is tagged out; that is, touched by a fielder's hand holding a live ball while in jeopardy, such as while not touching a base;
- he passes a base without touching it and a member of the defensive team properly executes a live ball appeal;
- he commits interference, such as when he contacts a fielder playing a batted ball, or when he contacts a live batted ball before it passes a fielder other than the pitcher;
- he strays more than three feet (.91 meters) from his running baseline in attempting to avoid a tag;
- he passes a preceding runner who is not out;
- he is touched by a fair ball in fair territory before the ball has touched or passed an infielder. The ball is dead and no runner may score, nor runners advance, except runners forced to advance. EXCEPTION: If a runner is touching his base when touched by an Infield Fly, he is not out, although the batter is out;
- he intentionally abandons his effort to run the bases; or
- he runs the bases in reverse order in an attempt to confuse the defense or to make a travesty of the game.
In baseball statistics, each out must be credited to exactly one defensive player, namely the player who was the direct cause of the out. When referring to outs credited to a defensive player, the term putout is used. Example: a batter hits a fair ball which is fielded by the shortstop. The shortstop then throws the ball to the first baseman. The first baseman then steps on first base before the batter reaches it. For this play, only the first baseman is credited with a putout, while the shortstop is credited with an assist. For a strikeout, the catcher is credited with a putout, because the batter is not out until the pitched ball is caught by the catcher. (If the catcher drops the third strike and has to throw the batter-runner out at first base, the first baseman receives the putout while the catcher receives an assist.) When an out is recorded without a fielder's direct involvement, such as where a runner is hit by a batted ball, the fielder nearest to the action is usually credited with the putout.
Although pitchers seldom get credited with putouts, they are credited with their role in getting outs through various pitching statistics such as innings pitched (a measure of the number of outs made by the pitcher, used in calculating his ERA) and strikeouts.
Further reading 
- Official rules for batters, including when the batter is out
- Official rules for runners, including when the runner is out