What Is a “Reorg” (Reorganization) in Blockchain Terminology?

A reorg, or reorganization, occurs when the network nodes switch from one chain tip to a different, longer chain tip, effectively replacing a portion of the transaction history. This happens when a miner or group of miners successfully mines a longer chain in secret and broadcasts it to the network.

In the context of a 51% attack, the attacker executes a reorg to replace the chain containing their initial payment (Transaction A) with their private chain containing the self-payment (Transaction B), enabling the double-spend.