A valid block that is no longer part of a main chain. This usually happens when two or more blocks of the same height are produced at the same time. When one of them becomes a part of the main chain, others are considered “orphaned”. Orphans may also occur when the blockchain is forked due to an attack (see 51% attack ) or a bug. Then a chain of several blocks may become abandoned. In the event that the blockchain is forked, an orphan block is on the chain that is abandoned for the main chain.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc