Immutable backups are backup copies that cannot be modified or deleted for a defined retention period. Once written, the data is locked against changes, even by administrators, which helps protect recovery points from accidental deletion, insider abuse, and ransomware encryption.
They matter because modern attacks often target backups first. If an intruder can erase or corrupt recovery data, the victim may be forced to rebuild systems from scratch or pay a ransom. Immutable storage, object-lock features, and write-once retention controls reduce that risk by keeping at least one clean copy available. In practice, defenders use them alongside network segmentation, offline copies, and regular restore testing so that recovery works when it is needed most.