What Is a “Reorg” (Reorganization) of the Blockchain?

A blockchain reorganization (reorg) occurs when a node switches from one chain tip to another, typically a longer, valid chain that was mined and propagated by another miner. This causes previously confirmed blocks on the shorter chain to become orphaned and their transactions to be reverted to the mempool.

Reorgs are a natural, though rare, part of Proof-of-Work consensus, but deep reorgs can be a sign of a 51% attack or network instability.