Questions about the client-local phenomenon when an Ethereum client discovers a new chain longer in terms of difficulty than its current chain.
Adapted from bitcoin.it:
The term "blockchain reorganization" is used to refer to the situation where a client discovers a new difficultywise-longest well-formed blockchain which excludes one or more blocks that the client previously thought were part of the difficultywise-longest well-formed blockchain. These excluded blocks become orphans.
Chain reorganization is a client-local phenomenon; the entire Ethereum network doesn't "reorganize" simultaneously.