Chain Reorganization (Reorg)
When nodes abandon one version of recent blocks for a longer competing version — routine when brief, alarming when deep.
A reorganization happens when the network switches from one recent chain of blocks to a different, competing chain — typically because two valid blocks were found near-simultaneously and one branch ultimately won. Short reorgs (one block) are a normal part of probabilistic-finality chains and resolve quickly.
Deep reorgs are serious: they mean transactions thought confirmed get undone, which is exactly what a 51% attack engineers to enable double-spends. This is why exchanges require more confirmations from chains with weaker security, and why deterministic finality (which makes reorgs impossible past a checkpoint) is a selling point of BFT-style chains. Reorg depth is a practical security signal.