An individual brought in at short notice to replace a member of staff, a player in a sporting team, etc.
Arising from or used in an emergency.
A situation which poses an immediate risk and which requires urgent attention.
The department of a hospital that treats emergencies.
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 person who is not good enough to be considered proficient but may be useful as a backup.