Orphan Block
An "orphan block" is a well-formed block with valid proof of work which has been rejected by miners and does not form part of the Block chain. As the block in question has not been accepted, the block reward and the transaction fees are no longer spendable on the difficulty wise-longest and well-formed blockchain. This phenomenon must be taken into account by mining pools that use any payout strategy other than "proportional".
An orphan block often originates when two miners produce a block around the same point in time. The time delay in propagating and accepting a block leads to Bitcoin SV nodes encountering scenarios where they need to select which of multiple blocks to accept as the most legitimate to include next in the chain. After a block is selected by the network, then the other block is considered an orphaned block. Transactions in the orphaned block that were not included in the successful block are re-included into the mempool as membership candidates for future blocks.
In other instances, orphan blocks may be generated when malicious nodes seek to reverse transactions by generating alternative desired blocks. This they hope to accomplish through the utilization of large amounts of hash power (see Attacks on Bitcoin). However, attacks intended to cause the deliberate orphaning of blocks are extremely resource-intensive and are largely infeasible. The blocks they generate generally end up being orphaned.
Conceptually, calling the above type of block an ‘orphan’ block is not the most apt description since the block does have a parent. Indeed, in the Bitcoin source code and in more technical discussions, orphan blocks and stale blocks are two separate concepts: both are not part of the longest valid chain, but in an orphan block it is excluded because the parent is unknown, whereas in a stale block it is because that part of the chain is known to no longer be longest. However, in general discussions people almost always mean "stale block" but say "orphan block".