Replication isn't a backup, because if you accidentally delete a file, and that deletion is immediately replicated, then you can't get that file back.
Backups are a specific point in time.
Backups are a specific point in time.
Are snapshots backups? snapshots on raid? snapshots on replicated disks?
Snapshots can be backups depending on where they are stored usually not if stored locally. For example, RDS snapshot is backup for database going down but not account being deleted or region destroyed. Generally, snapshots are way to make backups to more durable medium.
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.
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