Chain Reorganization
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."
Related Terms
Block
→A collection of transactions confirmed on the blockchain, each block building upon the previous one.
Confirmations
→The number of blocks added to the blockchain since a transaction was included.
Orphan Block
→A valid block which is not included in the current longest blockchain and thus does not contribute to the consensus protocol.
Majority Attack
→A potential attack on a blockchain network where a single entity gains control of the majority of the network's hash rate.
All terms and definitions may update as the Cryptionary improves.
