1 [mass noun] help or support:no police backup could be expected
someone or something that can be called on if necessary; a reserve:I’ve got a security force as backup [as modifier]:a backup generator
2 Computing a copy of a file or other item of data made in case the original is lost or damaged:make a backup of any important files [as modifier]:a backup copy
[mass noun] the procedure for making backup copies of files or other items of data:automatic online backup
3North American an accumulation of something caused by a blockage, as in traffic or water:there are long backups on all routes
