finality

Finality is the assurance that once a transaction is settled, it can no longer be reversed or undone. In everyday payments this is easy — a bank can declare a transfer final. On a decentralized blockchain with no central authority, finality is subtler: because the chain can occasionally fork, you need to know when a confirmed transaction has become safe enough that no realistic chain rearrangement could ever erase it.

Proof-of-work chains offer what is called probabilistic finality. A transaction is never finalized by a single stroke; instead, each new block stacked on top of it makes reversing it exponentially harder, because an attacker would have to outpace the entire honest network's work to rebuild that many blocks. The deeper your transaction is buried, the safer it is — which is why people wait for several confirmations before treating a large payment as done.

Many proof-of-stake chains offer stronger, faster economic finality instead. Validators explicitly vote to finalize blocks, and once a supermajority has done so, reversing those blocks would require attackers to get their staked deposits destroyed by slashing — a guaranteed, enormous loss. This turns finality from 'probably irreversible after waiting' into 'provably irreversible unless attackers torch a fortune,' giving faster, firmer settlement.