Crypto glossary
Immutability
Immutability means confirmed blockchain records cannot be changed or deleted, which makes the ledger trustworthy and mistakes irreversible.
Each block contains the hash of the previous one, so altering an old transaction would change every hash after it. On a public network, rewriting history would also require redoing the proof of work or controlling enough stake to overpower honest participants, and convincing thousands of nodes to accept the new version. In practice, deeply buried transactions are treated as permanent.
Immutability is what lets strangers rely on balances, token ownership and smart contract code without trusting the operator of a database. It also gives blockchains their value as timestamping and audit tools: a hash recorded on Bitcoin years ago proves a document existed at that time.
The property is social and probabilistic, not absolute. Recent blocks can be reorganised, small chains can be attacked, and communities have reversed history by hard fork, as Ethereum did after The DAO hack in 2016. Immutability also cuts both ways: payments sent to a wrong address or drained by a scam generally cannot be undone, and data written on-chain cannot easily be erased to satisfy privacy laws.
Example
A user who sends USDC to a mistyped address cannot ask the network to reverse it; unless the recipient returns the funds voluntarily or the issuer freezes them, the transfer stands.