Immutability
Immutability means that once data is recorded on the blockchain it is practically impossible to alter or delete.
In depth
It is not a law of physics but a consequence of design: hash-linked blocks, thousands of copies and the cost of redoing proof of work. Changing a past record would require overpowering the entire network’s honest participants. In practice, transactions a few blocks deep are considered permanent.
Why it matters
Immutability is why crypto payments are final — and why sending to the wrong address cannot be undone.