All starting players (there are either 9 or 10, depending on whether a designated hitter is being used) can be taken out of the game and put back into the game one time (this is called “re-entry”). A player can only be listed as a starting player, however, if he is present at the game site when the lineup cards are exchanged by the coaches.
A starting player who re-enters has to go back into the same batting position as where he started.
In a situation where a pitcher has become injured or ill, the starting pitcher may not re-enter if he has not pitched to one batter, or if more than eight warmup pitches are used as the offense makes the switch.
A similar rule is the one that allows courtesy runners for the pitcher and/or catcher.
[OBR: a replaced player is out of the game unless he is a pitcher who has not met his obligation to face a batter or retire the side; that pitcher must re-enter and meet his obligation.]