noun [U,C]
1 extra help or support that you can get if necessary:
The police had back-up from the army. * We can use him as a back-up if one of the other players drops out. * a back-up power supply
2 (computing) a copy of a file, etc. that can be used if the original is lost or damaged:
Always make a back-up of your work. * a back-up copy
