Chain Reorganization
blockchain · security
A network event where the active chain tip switches to a different branch with more accumulated work.
- Also known as
- Chain Reorg, Reorg
1.definition
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.
2.implications
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.
Related terms
Block
→A batch of valid transactions added to a blockchain, linked to the previous block by a cryptographic hash.
mining · blockchain · security
Confirmations
→How many blocks have been added starting with the block that includes a transaction.
security · blockchain
Orphan Block
→A block that cannot currently extend a node's active chain, often because its parent is unknown or a competing branch lost the proof-of-work race.
mining · blockchain
Majority Attack
→A proof-of-work attack in which coordinated hash power can outpace the honest chain and reorganize recent valid history.
security · consensus
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