Orphans

"Usually when people say "orphan block", they mean a Stale Block, which is a well-formed block which is no longer part of the difficultywise-longest and well-formed blockchain. The Block Reward in a stale block is no longer spendable on the difficultywise-longest and well-formed blockchain; therefore whoever mined that block does not actually get the reward (or the transaction fees). This phenomenon must be taken into account by mining pools that use any payout strategy other than "proportional".

Conceptually, calling the above type of block an orphan block doesn't make any sense, since it does have a parent. Indeed, in the Bitcoin source code and in more technical discussions, orphan blocks and stale blocks are two separate things: both are not part of the longest valid chain, but in an orphan block it is 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"."