alt Hacker NewsA backup is something that will functionally replace the original should the original fail, regardless of how the original failed. For data, this means that the restore process is part of the backup.
Snapshots are not backups. Snapshots on RAID are not backups. Snapshots on replicated disks are probably backups, so long as the disks being replicated to are not inside the same case/building/city/continent (pick your risk suitably) and you're not able to delete the snapshots from the machine hosting the originals.
The second SIM in my phone provides a backup for my primary service provider, so long as I keep it activated. The torch in my pocket is a backup for the lighting in my house, so long as I keep it charged. My data in tarsnap is a backup, so long as I'm able to restore it. Which means data in tarsnap isn't a complete backup on its own: unless I'm able to recover the encryption key, I don't actually have a backup.
Indeed, so if it's on the same continent/building/room is it a backup? depends why you need to restore it. You can't tell whether something is a backup until it's restored, it's Schrodinger's backup.
A snapshot is a backup if a user deletes/edits their file and wants the old version. Raid is a backup if you're recovering from 1 disk failing