Immutable (Immutability)
In short: Immutability is the property of data that cannot change over time. On a blockchain, it means that once a transaction is recorded and confirmed, it cannot…
Immutability is the property of data that cannot change over time. On a blockchain, it means that once a transaction is recorded and confirmed, it cannot realistically be edited, reversed or deleted.
Why it matters
Immutability lets people trust a shared record without trusting a single operator. Each block contains the hash of the block before it, so altering an old transaction would break the link to every later block, and an attacker would have to rebuild all of them and get the network to accept the new version. The same property means mistakes are permanent: a payment sent to the wrong address cannot be undone by the network. Because a transaction becomes harder to reverse with every new block, exchanges wait for a set number of confirmations before crediting a deposit.
Example
Suppose a payment is included in the newest block of a proof-of-work chain. For a short time, a competing block could still replace it. After several more blocks are built on top, reversing the payment would mean redoing all of that work faster than the rest of the network, so in practice the record is treated as final.