Snapshot

A snapshot captures the state of a system or virtual machine at a specific point in time so you can roll back to exactly that state shortly afterward. It exists mainly to provide a safety net around risky changes - an update, a configuration change - before they are actually made.

In practice a snapshot is usually taken manually or automatically right before a planned change and kept around for a few hours or days afterward. Technically, a snapshot only records the differences from the original state, not a full, independent copy of the data.

The most expensive and most common mistake in practice is treating a snapshot as a backup. A snapshot typically sits on the very same storage device as the original data and depends on that device being available - if the storage fails or gets encrypted, the snapshot disappears right along with the original. Snapshots are also usually meant to be short-lived and get deleted automatically on a schedule, which makes them useless for restoring something from weeks or months ago.