something that you can use to replace something that does not work or is lost:
Always have a backup plan.
a backup generator
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.
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
4 [countable] American English
someone who will play in a sports team if one of the other players is injured or ill [= reserve]:
a backup goalie