Backup
A separate copy of image files kept so that loss of the original does not lose the work.
Measured in
count
In depth
A copy on the same drive is not a backup, and a mirrored RAID is not a backup either — both fail together against deletion, corruption or theft. A real backup is independent, verified and restorable, and its only meaningful test is whether a file has actually been recovered from it recently.
Examples in practice
A photographer restores a folder deleted six months ago from a backup, confirming the system actually works.