immutability

Immutability is the property that once data has been recorded on a blockchain and confirmed, it cannot be quietly changed or deleted. The record becomes write-once: you can always add new entries on top, but you cannot reach back and edit what is already settled. This is what lets a blockchain serve as a permanent, trustworthy history that everyone can rely on.

It does not come from a law or a promise — it comes from how the blocks are stitched together. Each block carries the hash (fingerprint) of the previous block, so the blocks form a tightly linked chain. Altering an old transaction would change that block's fingerprint, which would break the link in the very next block, and the next, all the way to the present. To make the edit stick, an attacker would have to redo the work of every block since — and out-race the entire honest network doing it, which on a large chain is wildly impractical.

The practical upshot is that the deeper a transaction is buried under later blocks, the more final it becomes. After enough confirmations, reversing it is effectively impossible. This is why immutability is prized for things like financial records, property titles, and audit trails — but it is also why mistakes are unforgiving: a payment sent to the wrong address cannot simply be undone.