Immutability

Immutability means "can't be changed". Once a transaction is buried in a blockchain under enough later blocks, editing it would mean redoing all that work and outvoting the honest majority of the network. On a large chain, that's practically impossible.

It's the source of crypto's trust: nobody, not even a government or the developers, can quietly rewrite who paid whom. It's also unforgiving: send coins to the wrong address and there's no undo button. See bricked funds.