Chain Reorganization

A chain reorganization (or "reorg") occurs when a blockchain replaces one or more recently confirmed blocks with an alternative set of blocks that represents a longer or more valid chain. This can happen naturally when two miners find a block at nearly the same time, with the network eventually converging on one chain. Malicious reorgs (via 51% attacks) can reverse confirmed transactions. Deep reorgs are rare on large networks like Bitcoin due to the enormous hash power required.