A 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.

Backup describes what they are for. There are many ways to do backups. The main thing is that backups are archive stored away from danger. There are different kinds of danger and different need for protection.

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.