While it may sound intricate, immutability is a sophisticated term that signifies something that cannot be modified. The "state" or "existence" of an object or data, such as the transaction records on a public blockchain, remains constant and cannot be altered once created.

The antonym of immutable is changeable, alterable, or variable.

When discussing blockchains, particularly Bitcoin, immutability is a fundamental and essential characteristic, which is primarily addressed in two contexts.

Firstly, immutability pertains to a blockchain's capacity to preserve its transaction history in a manner that cannot be altered, replaced, or manipulated by any entity, such as a government, another business, or even miners validating transactions for the network. Bitcoin's hash function enforces the network's append-only block creation feature. When a block is added to the chain, the hash of the preceding block is incorporated into the newly created block. If the hash of the previous block were modified in any way, the hash of the current block would also change, thereby invalidating the entire block. This ensures that the transaction data on the Bitcoin network is immutable or unchangeable. Any alteration to one block would impact the subsequent block, and a modification to that block would affect a third block, and so forth. Changing one block necessitates alterations to all following blocks.

Secondly, again referencing the Bitcoin network, Bitcoin's proof-of-work (PoW) consensus protocol ensures that the network's monetary ruleset remains unchangeable. Thousands of Bitcoin nodes operate independently, some managed by large corporations or mining pools, while others are run by individuals in a peer-to-peer network. All nodes share a collective voice, but no single node serves as the master or a point of failure. Each node operates the same code and agrees on the same rules governing the network. Additionally, any proposed changes to the network must be voted on and agreed upon by all nodes. Consequently, blockchain-based voting systems cannot be altered, nor can votes be rescinded.

This element of shared responsibility and transaction validation, along with block creation and consensus on rule changes, instills trust among network participants that Bitcoin's monetary policy cannot be modified by a single node but requires the consensus of thousands of nodes, thereby preserving the structure as much as possible.