n
[C]
something that you can use to replace something that does not work or is lost
　Always have a backup plan.
　a backup generator
[C]
a copy of a computer document, program etc, which is made in case the original becomes lost or damaged
　Make a backup of any work you do on the computer.
[U]
people or things that can be used to provide support and help if they are needed
　Army units can only operate if they have sufficient backup.
　a backup team
[C]AmE
someone who will play in a sports team if one of the other players is injured or ill
= reserve
　a backup goalie