Immutability means that data recorded on a blockchain cannot be altered or deleted after it has been confirmed.
In computer science, an immutable object is one whose state cannot be modified after it is created. In the context of blockchain technology, immutability refers to the property that transaction data, once confirmed and added to the chain, cannot be changed retroactively.
Each block in the chain contains a cryptographic hash of the previous block, a timestamp, and transaction data. Because the hash of each block depends on the contents of the block before it, changing any historical record would change that block's hash, and therefore invalidate every subsequent block.
This architecture means that the immutability of a blockchain is not absolute, but probabilistic: the deeper a transaction is buried under subsequent blocks, the harder it becomes to alter, and the more confident participants can be that the record is permanent.
Immutability can enhance trust in shared record-keeping systems where multiple parties who don't fully trust each other need to agree on a single source of truth.
In business contexts, this can reduce the time and cost of audits, since verifying information becomes straightforward rather than requiring reconciliation across multiple internal databases.
It can also provide clarity in business disputes, support compliance and reporting, and give organizations a complete historical record of processes without the risk of records being altered after the fact.
While immutability is a core feature of most public blockchains, the data stored on them is not fully immune to attack.
On Bitcoin, mounting such an attack would require enormous hardware investment and ongoing electricity costs, making it extremely difficult in practice. However, smaller proof-of-work blockchains with lower total hash rates are more vulnerable, since the cost of acquiring majority hash rate on a minor network can be low enough that attacks become financially viable. The degree of immutability a blockchain provides is therefore tied to the size and economic strength of its network, making it more of a spectrum than an absolute property.