What Is ‘Finality’ in Blockchain Transactions and How Does a 51% Attack Compromise It?

Finality refers to the guarantee that a confirmed transaction cannot be reversed or altered. In PoW, finality is probabilistic; the deeper a block is buried, the more secure it is, but a reversal is theoretically possible.

A 51% attack compromises this by forcing a deep reorganization, effectively reversing transactions that were previously considered final. This destroys the economic trust in the network's transaction history.