What is an immutable backup?

In this age of Ransomware , backups and protection of backups is critical. One question asked in any review of database protection against attacks is whether the backups are immutable?

Definition of immutable backups:  Immutable backups means that the backup is unchangeable , never deleted or can be changed.   

This feature is very important against accidental or intentional deletion of backups. A related term is WORM - (Write once read many) - essentially locks backup data from random changes. If WORM enabled  storage is also part of a  least privilege security architecture - based on zero trust , than this adds extra protection. 

Once you begin to delve into immutable backups you get into all sorts of questions - such as : You may have an immutable backup - but if your backup has been infected with malware and you restore back into the Production system - what will happen? So how does a DBA actually protect the systems from the backup architecture itself

Immutable backups themselves are very useful - but further questions need to be asked about processes around backups , which may be corrupted and mask other activity.:

1)Are the backups jobs  being monitored? Not for backup failure  but the deletion of the job.  

2)How are notifications processed ?