Glossary
Immutability
The practical guarantee that data already buried in a blockchain cannot be altered without redoing every block that came after it.
Strictly speaking, no blockchain is immutable — given infinite resources, an attacker could reorganize it. In practice, the cost of re-mining or re-staking thousands of blocks fast enough to overtake the honest chain is so large that recent history can be treated as final.
Immutability is what makes blockchains useful for settlement: once a transaction has enough confirmations, neither the sender, the recipient, nor the protocol developers can quietly undo it.