Definition
A block reorganization, or reorg, happens when a node abandons one or more blocks at the tip of its chain and adopts a competing branch that carries more accumulated proof-of-work. Bitcoin nodes always follow the most-work chain, so when two miners find blocks at nearly the same height a temporary fork forms; the branch that next extends fastest wins, and the losing blocks are discarded.
How reorgs occur
Block propagation across the global network is not instant, so brief competing tips are normal. Most reorgs are one block deep and resolve within minutes when the next block extends one branch. Transactions in the orphaned block return to the mempool to be re-mined, unless they were already included in the winning branch.
Why depth matters
Shallow reorgs are routine and harmless to confirmed transactions, which is precisely why merchants wait for several confirmations. A deep reorg, one that rewrites many blocks, would require an attacker to out-mine the honest network over a sustained period, the same capability behind a 51% attack. No such deep reorg has ever succeeded on Bitcoin's mainnet, though smaller chains have suffered them.
Reorgs are the mechanism behind Bitcoin's probabilistic finality: a transaction is only ever as final as the work piled on top of it is hard to redo. Blocks discarded in a reorg become orphan or stale blocks, and a reorg deep enough to undo a payment is the practical aim of a double spend.
In Simple Terms
A block reorganization, or reorg, happens when a node abandons one or more blocks at the tip of its chain and adopts a competing branch…