Immutability
The property that once data lands on a blockchain, no one can quietly alter or erase it.
Immutability means past blocks are effectively permanent. Each block carries a hash of the one before it, so rewriting an old record would break every block after it. On a chain like Bitcoin, an attacker would need to redo enormous amounts of work to forge that history.
The benefit is a tamper-evident record no single party can edit. The downside is that mistakes stick. A typo in a transaction or a bug in a smart contract cannot be undone, only worked around. Immutability is closer to 'practically impossible to change' than literally eternal.
Related terms
A shared digital ledger that records transactions in linked batches, copied across many computers and hard to alter.
HashA fixed-length fingerprint produced by running data through a one-way function; any change yields a completely different output.
51% AttackAn attack where one party controls most of a network's mining power and can rewrite recent transactions.