What Is a ‘Reorg’ (Reorganization) and How Is It Used in a 51% Attack?

A reorg is a temporary event where the network discards one chain of blocks and replaces it with a different, longer chain that has been secretly mined. In a 51% attack, the attacker secretly mines a longer chain that omits the transaction where they sent coins to an exchange.

When this longer chain is released, the network switches to it, reversing the original transaction and enabling the double-spend.