What Is the Concept of “Finality” in a Blockchain and How Does a 51% Attack Challenge It?

Finality refers to the guarantee that once a transaction is recorded in a block, it cannot be reversed, altered, or removed from the blockchain. In Proof-of-Work, finality is probabilistic, increasing with each subsequent block (confirmations).

A 51% attack directly challenges this by enabling the attacker to create a longer, alternative chain that excludes the original transaction, effectively reversing it. This breaks the fundamental assumption of immutability and finality that underpins the network's trust.