Immutable means permanent or unable to be changed, altered, or edited.
This word is often used to describe one of the key aspects or properties of a public and properly decentralized blockchain network.
The choice of immutable to express this concept of unchangeable likely comes from computer programming parlance.
Immutability, like permissionless, is an important crypto primitive.
After all, the core utility of a blockchain is to act like an accessible digital ledger — or a permanent record of transactions, details, and related data.
If such a digital ledger was prone to change, or if it was possible to rewrite the past or alter previous transaction details, then those changes could call into question the validity of the entire ledger.
Blockchains work because one block of data refers to a previous block. In other words, a blockchain’s data is not legitimate because of one block of confirmed transactions, but because of the entire history of confirmed transactions that proceed that one block.
From a security perspective, the idea of immutability is also fundamental. Chains that have a long history of confirmed transactions ultimately have a higher level of security because it would take more computational resources to attack the chain in order to gain control and rewrite transaction data.
Despite the importance of immutability, or maybe because it is such a bedrock piece of crypto, immutability can be controversial.
The early days of Ethereum has one instance of developers rolling back Ethereum’s blockchain in order to secure funds stolen from an early attempt at a Decentralized Autonomous Organization.
The decision to rollback the blockchain to essentially undo a digital heist caused a rift in the community (eventually culminating in two chains Ethereum and Ethereum Classic) but also called into question the core concept of immutability.