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.
A substitute; a replacement.
To substitute for; to replace; (theater) to serve as an understudy.
To make one of a party in a bet or other speculation.
To take a side in a dispute.