A network event where the active chain tip switches to a different branch with more accumulated work.
A reorg happens when two valid chains compete and the network adopts the one with more accumulated proof-of-work. Transactions in the orphaned blocks return to the mempool if not included in the new chain.
Small reorgs (e.g., 1–2 blocks) can occur naturally when blocks are found close together. Deep reorgs are rare and may indicate an attack or severe network partition. During a reorg, previously seen confirmations are reduced, so merchants often wait for multiple confirmations for high-value payments.
"A payment with 1 confirmation can return to 0 confirmations if a competing chain becomes longer and the block containing the payment is orphaned."
"Exchanges often require 6 confirmations for BTC deposits to mitigate the risk of reorgs affecting credited balances."
All terms and definitions may update as the Cryptionary improves.