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Immutability means inability to change. In computer science, an immutable object is an object whose state cannot be altered after its creation.

Immutability is one of the key characteristics of Bitcoin and blockchain technology. Immutable transactions make it impossible for any entity (for example, a government or a company) to manipulate, replace or falsify data stored on the network.
Since all historical transactions can be checked at any time, immutability allows a high degree of data integrity.

The immutability of public blockchains can improve the current system of trust and control, and can reduce the time and cost of audits as verifying information becomes much easier or effectively redundant.

Immutability can also increase the overall efficiency of many companies by providing them with the opportunity to maintain a complete historical record of their business processes. Immutability can also provide clarity to many corporate disputes, as it allows for a verifiable and shared source of truth.

While immutability is one of the main benefits of Bitcoin and blockchain technology, the data stored on blockchains is not completely resistant to vulnerabilities: if an attacker were able to accumulate the majority of the network's hash rate - hashrate, could alter otherwise immutable data in an attack that was called 51% attack.
In such a scenario the one who holds 51% of the hashrate could prevent new transactions from getting confirmations or even reverse transactions completely. Can it also happen to Bitcoin? Yes, but it requires monstrous hashing power, very expensive hardware, and significant amounts of electricity.

On the other hand, the networks Proof of Work with lower hash rates they are vulnerable to such an attack, as gathering the necessary amount of hashing power to attack the network is not such an unreasonable undertaking.