An event where a one chain becomes longer than the one currently being worked on. All of the blocks in the old chain that are not in the new one become orphan blocks and their generations are invalid. Transactions that use the newly-invalid generated coins also become invalid. However, this is only possible in large chain splits because generations cannot be spent for 100 blocks. Usually, newly received blocks are extending the existing main chain. Occasionally, a couple of blocks of the same height are produced almost simultaneously and for a short time some nodes may see one block as a tip of the main chain which will eventually be replaced by a more difficult block(s). Each transaction in the orphaned blocks either become invalid (if already included in the main chain block) or becomes unconfirmed and moves to the mempool . The number of confirmations for transactions may change after a re-org, and transactions that are not in the new chain will become “0/unconfirmed” again. If a transaction in the old chain conflicts with one in the new chain (as a result of double-spending), the old chain becomes invalid. In the case of a major bug or a 51% attack , reorganization may involve reorganizing more than one block.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc