Orphan Block
A valid block that loses the race to be part of the main chain when two blocks are found at nearly the same time.
Sometimes two miners find a valid block at almost the same moment and broadcast them to different parts of the network. Both are legitimate, but only one can survive. As subsequent blocks are built, the network converges on the chain that gains length fastest, and the competing block is left out. That discarded block is an orphan, and its reward does not count. Orphans are a normal, rare consequence of a distributed network with propagation delay. For home miners they explain the occasional disappearance of a reward that briefly appeared, and they are nobody fault; the protocol simply picks one valid history and moves on.