An individual who substitutes for another to perform one or more tasks.
A substitute batter; one who pinch-hits.
An aggressive batsman brought on to score runs quickly, even at the risk of losing his wicket.
A person who is not good enough to be considered proficient but may be useful as a backup.
A person who plays second string. A person who is kept on a sports team as a backup in case a "first-string" player is unavailable.