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

Finality refers to the guarantee that a transaction, once confirmed, cannot be reversed or altered. In PoW, transactions achieve probabilistic finality after a sufficient number of subsequent blocks are added.

A 51% attack violates this by creating a longer, secret chain that includes a double-spend transaction, then releasing it to the network, forcing a chain reorganization (reorg). This reverses the original transaction, nullifying its perceived finality.