What Is a “Reorg” (Reorganization) in the Context of a 51% Attack?

A reorg, or reorganization, occurs when a node on the network switches from following one chain of blocks to following a different, longer chain that has been broadcast. In a 51% attack, the attacker secretly mines a private chain that is longer than the public chain, containing the double-spent transactions.

Once the private chain is broadcast, honest nodes recognize it as the canonical chain due to the "longest chain rule" and reorganize their ledger, effectively reversing the original, legitimate transactions.