An orphan block (block A) is a valid block, but it is not a part of a blockchain. An orphan block is created when two miners find a valid block (block A and block B) at around the same time and broadcast them both to the network.

The orphan block (block A) is at first accepted (confirmed) by some nodes (which are usually geographically closer to the miner) but the other block (block B) accumulates more proof of work—it is is accepted by more nodes and becomes a parent block for the next one. The orphan block (block A) is then marked as invalid because it is not part of the longest chain. The miners do not receive a reward for such a block.