What Is a ‘Reorganization’ and How Does It Relate to Hash Rate?

A blockchain reorganization, or 'reorg,' occurs when a temporary fork in the blockchain is resolved, and the nodes switch from one chain to another, typically the one with the most cumulative work (longest chain rule). A deep reorg is a red flag for a 51% attack, as it indicates an attacker successfully built a longer, secret chain with their concentrated hash rate and then broadcast it to replace the honest chain, enabling double-spending.