Immutability means that once data is written to a blockchain, it stays there. No edit button exists. No administrator can quietly revise an old record. Each block carries the hash of the block before it, and changing anything in a past block breaks every hash that follows. The network would reject the altered chain and keep the original.

This property is why blockchains work for records that need to be trusted over long periods. Property titles, supply chain logs, and payment history all benefit from a ledger nobody can rewrite. But immutability has a darker side. A bug in a smart contract cannot be patched. A transaction sent to the wrong address cannot be recalled. The DAO hack in 2016 forced Ethereum to choose between immutability and recovering stolen funds. The community chose recovery, split the chain, and Ethereum Classic kept the original history. Neither side was obviously wrong.

What immutability does and does not mean

• Past blocks resist alteration — yes, at prohibitive cost
• Data inside a block cannot be changed — yes
• Mistakes can be reversed by forking — technically possible, but contentious
• Immutability equals truth — no, only that the record was agreed upon

A blockchain records what the network accepted. Whether that matches reality is a separate question.