Definition
A chain reorganization, or reorg, happens when a node drops blocks it had previously accepted at the tip and switches to a different branch carrying more cumulative proof of work. Bitcoin nodes always follow the chain with the greatest total work under the longest-chain rule, so when a heavier competing branch appears, the node re-orders its recent history to match.
Why Reorgs Happen
Most reorgs are routine and harmless. When two miners find blocks at nearly the same height, propagation delay leaves parts of the network temporarily following different tips. Whichever branch the next block extends wins, and nodes on the losing side reorganize. The displaced blocks become stale blocks, and any transactions unique to them return to the mempool to be re-mined. A one-block reorg is common and usually invisible to users.
When Reorgs Become Dangerous
Deeper, deliberate reorgs are the mechanism behind several attacks. An entity controlling enough hashrate can privately build a longer branch and release it to reverse confirmed transactions, the core of a 51% attack or double-spend. This is precisely why merchants wait for multiple confirmations: each additional block makes a reorg of that depth exponentially more expensive. Six confirmations is the long-standing convention for high-value settlement.
Reorgs are inseparable from how Bitcoin reaches eventual consensus, not a defect in it. Related reading includes orphan blocks, the Finney attack, and race attacks.
In Simple Terms
A chain reorganization, or reorg, happens when a node drops blocks it had previously accepted at the tip and switches to a different branch carrying…