Chain Reorganization
An event where a blockchain node switches from its current chain to a different, now-longer or heavier competing chain, discarding one or more recently added blocks.
A chain reorganization, often called a "reorg," occurs when a node discovers a competing version of the blockchain that's now considered more valid than the one it was previously following — typically because that competing chain has become longer (proof-of-work) or has accumulated more validator attestations (proof-of-stake) — and switches to it, discarding one or more recently added blocks from its previous chain in the process. Any transactions that were only in the discarded blocks return to an unconfirmed state, needing to be re-included in a subsequent block on the now-canonical chain.
Small, shallow reorgs of just one or two blocks happen periodically on most proof-of-work chains as a normal consequence of near-simultaneous block discovery by different miners, and are generally harmless since the small number of affected transactions simply get re-confirmed shortly after. A deep reorg affecting many blocks is far more unusual and concerning, since it implies an attacker had enough resources to build a longer competing chain in secret — this is the mechanism underlying a 51% attack (see Kaavon's "51% Attack" glossary definition), which is precisely why services commonly wait for multiple confirmations before treating a transaction as settled.
Examples
- A shallow, one-block chain reorganization happens periodically on proof-of-work chains simply due to near-simultaneous block discovery, with little practical consequence.
- A deep chain reorganization affecting many blocks is a hallmark of a successful 51% attack, which is why exchanges wait for multiple confirmations before treating large deposits as final.