Orphan Block

An orphan block is a block that doesn't have a known parent in the longest block chain.

Not to be confused with 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".