While sounding complex, immutability is a fancy word that describes something that cannot be changed.  The “state” or “being” of an object or of data in question, say like a public blockchain’s record of transaction data, stays the same and can’t be altered after creation.

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

When talking about blockchains, and Bitcoin specifically, immutability is a core and critical feature, and it’s talked about primarily in two ways.

In the first way, immutability refers to the ability of a blockchain to maintain its transaction history in a way that it can’t be altered, replaced or manipulated by some entity, like a government or another business or even miners working to validate transactions for the network.

Bitcoin’s hash function enforces the network’s append-only block creation characteristic.

When a block is added to the chain, the hash of the previous block is embedded in the newly created block.

If the previous block’s hash were changed in any way, the current block’s hash would also change, invalidating the entire block.

This makes the transaction data on the Bitcoin network immutable or unalterable.  Any changes to one block would affect the next block, and a change to that block would alter a 3rd block.  And so on.  Altering one block means all subsequent blocks would also need altering.

In the second way, and again referencing the Bitcoin network, Bitcoin’s proof-of-work (PoW) consensus protocol makes it so that the network’s monetary ruleset can never be changed.

Thousands of Bitcoin nodes run independently, some operated by large businesses or miner pools, and others run by individuals, in a peer-to-peer network.

All nodes have a common voice, but no single node will act as the master or as a single point of failure.

All the nodes run the same code and they all agree on the same rules governing the network.

They all also must vote and agree to any changes proposed to the network.  Blockchain-based voting systems, then by extension, can’t be altered or votes removed.

This aspect of shared responsibility and validation of transactions, block creation, and agreement on rules changes provides trust to network participants that Bitcoin’s monetary policy can’t be changed by just one node but will require thousands of nodes to all agree.