What Is a Reorg (Reorganization) in Blockchain Terminology?

A reorg, or reorganization, occurs when the network temporarily or permanently switches from one chain of blocks to a different, longer chain. This typically happens when two miners find a block almost simultaneously, creating a temporary fork.

The network follows the "longest chain rule," and the shorter chain's blocks are discarded, effectively reversing the transactions they contained. A malicious reorg is the mechanism used in a 51% attack to reverse a target transaction.