Overview
Immutability is a core feature of blockchain technology. It ensures that once a transaction is confirmed and added to the chain, it is permanent and cannot be altered by any single entity.
How it's Achieved
Each block contains a hash of the previous block. If you change one transaction in an old block, its hash changes, which breaks the link to the next block, and so on. To change history, an attacker would have to re-mine all subsequent blocks, which is computationally impossible in a sufficiently decentralized network.
Importance
Provides trust, auditability, and security, making blockchain ideal for financial records and legal documents.