Announced into the game
A player has been announced into the game when his manager advises the home plate umpire that he is coming in as a substitute for another player. He is then allowed to take part in the play, while the player he has replaced can no longer return, except in special circumstances such as the use of a courtesy runner (now no longer allowed) or in the All-Star Game when certain players who have been substituted for may return at a later point.
The matter of whether a player has been announced or not is of particular importance in a situation when a manager decides to send a pinch-hitter, then changes his mind. For example, if the purported pinch-hitter is only standing in the on-deck circle, he has not yet been announced into the game, and the player he was set to replace may still come to bat as scheduled, or a second player can be used without this affecting the first player's eligibility at a later point in the game. However, as soon as the preceding at-bat ends and his manager advises the umpire he is coming in, that player is considered to have entered the game. If the opposing manager then changes his pitcher and the manager decides to reply by using a different pinch-hitter, the first pinch-hitter is credited with a game played, even though he may not have seen a single pitch, and cannot be used any further.
The same applies to a pitcher who is injured while warming up to enter the game. He is announced into the game when he walks to the pitcher's mound, and cannot be used further if he is then unable to pitch for any reason. In that situation, the pitcher is credited with a game played but not a game pitched.