Definition
A block reorg is a consensus-level process in which one or more recently added blocks are discarded and replaced by blocks from a competing chain. It occurs when the network’s fork choice rule selects a different branch of blocks as the canonical chain, causing the previous branch to be rolled back. During a block reorg, the set of accepted transactions in the affected blocks can change, even though the underlying protocol rules remain the same.
Block reorgs are closely tied to how a blockchain defines and reaches finality. Before a block has enough block confirmations, it is more vulnerable to being removed in a reorg if an alternative chain becomes preferable according to the consensus mechanism. As more blocks are built on top of a given block, the likelihood of a reorg affecting that block decreases, and its status in the chain becomes more stable.
Context and Usage
In practice, a block reorg reflects the dynamic nature of distributed consensus, where nodes may temporarily disagree on which block should follow the latest confirmed block. The process of block reorganization ensures that all honest participants eventually converge on a single shared history, even if short-lived forks appear. This makes block reorgs a normal but carefully monitored aspect of maintaining a consistent ledger.
The depth and frequency of block reorgs are important indicators of network health and security. Shallow, infrequent reorgs typically arise from benign timing differences in block propagation, while deeper reorgs can signal more serious issues with consensus or potential attacks. Understanding block reorgs helps clarify why multiple block confirmations are often required before a transaction is treated as effectively final.