Definition
Block reorganization is a consensus-level process in which a blockchain node replaces a sequence of previously accepted blocks with an alternative sequence that better satisfies the network’s fork choice rule. It occurs when the node learns about a competing chain segment that is considered more valid or has higher priority according to the protocol’s rules. The node then updates its local view of the canonical chain, effectively rewriting the most recent portion of its recorded history.
This process can cause some blocks to become orphan blocks, meaning they are no longer part of the main chain even though they were once accepted by some nodes. Block reorganization is typically limited to recent blocks and is closely related to the concept of finality, which describes when blocks are considered economically or probabilistically irreversible. While reorgs are a normal aspect of decentralized consensus, frequent or deep reorgs may indicate instability or attack conditions in a blockchain network.
Context and Usage
In practice, a block reorganization is triggered when different nodes temporarily extend different branches of the chain and later converge on a single branch. Each node independently evaluates competing branches and, following the fork choice rule, may reorganize its local chain to align with the branch recognized as canonical. This ensures that all honest nodes ultimately share a consistent view of the blockchain’s state.
The term “block reorg” is commonly used interchangeably with block reorganization in technical discussions among node operators and protocol designers. References to reorg depth describe how many blocks were replaced during the process, which is important for understanding how close recent blocks are to achieving finality. Nodes are designed to handle such reorganizations automatically, maintaining consensus even in the presence of temporary network splits or latency.