# What is a Backup?
> A backup is a separate, recoverable copy of data kept so it can be restored after loss — the baseline control against deletion, corruption, ransomware, and failure.
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## Content
Effective backup follows the 3-2-1 pattern: multiple copies, on different media or platforms, with at least one isolated from the system it protects. Modern threats make isolation and testability the critical properties — an online backup reachable by the same compromised credentials is not a recovery option; it is another target.
### Key points
- **Isolated copies:** Offline, immutable, or separately authenticated copies survive ransomware and credential compromise that reach everything else.
- **Restore testing:** A completed backup job proves data was copied; only a timed restore test proves it can come back within the RTO and RPO.
- **Important limitation:** Backups preserve data, not working systems — restoring operations also needs configurations, secrets, dependencies, and clean environments. A backup that silently failed or captured already-corrupted data protects nothing.
### Related terms
[Immutable backup]() · [Disaster recovery]() · [Cyber recovery]() · [Recovery point objective (RPO)]() · [Data retention]()
### Sources
[CISA, #StopRansomware Guide](https://www.cisa.gov/stopransomware) · [NIST SP 1800-11, Data Integrity: Recovering from Ransomware and Other Destructive Events](https://csrc.nist.gov/pubs/sp/1800/11/final)
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