Reorganisation
When a node switches to a competing branch of the chain that has more accumulated work.
A reorganisation, or reorg, happens when a node sees a competing branch of the blockchain that carries more accumulated proof-of-work than the one it was following, and so rewinds and switches to the heavier branch. Short reorgs of a block or two are a normal, self-healing part of how the network converges after two miners find blocks at nearly the same time. The abandoned branch blocks are simply discarded. Deep reorgs of long-settled history effectively never happen, which is why older transactions are treated as final and why a fresh block is not considered settled until several more blocks build on top of it. Nodes always follow the heaviest valid chain.