A copy of data kept separately so that information can be restored after loss, corruption, or an attack.
A backup is a copy of data that can be used to restore the original if it is lost, damaged, or encrypted by ransomware. A common guideline is the 3-2-1 rule: three copies, on two types of media, with one kept offsite or offline.
Backups are only as good as their restores. Regular restore testing is essential, and at least one copy should be isolated so that an attacker who reaches live systems cannot also destroy the backups.
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