Snapshot
A point-in-time, read-only copy of a filesystem or volume that lets you roll back changes instantly.
Snapshots on ZFS or Btrfs are near-instant and space-efficient because they only track changes. They're perfect for recovering from mistakes or ransomware, but because they live on the same pool they are not a substitute for offsite backups.
Related articles
ZFS
An advanced filesystem and volume manager offering data integrity, snapshots, compression and software RAID.
Backup
A separate, recoverable copy of your data kept to protect against loss, corruption or disaster.
RAID(Redundant Array of Independent Disks)
A method of combining multiple drives so they act as one, improving performance and/or protecting against drive failure.