Orphan Block

An orphan block, more precisely called a stale block, is a valid block that loses the race to become part of the canonical chain. It happens when two miners find a block at nearly the same height and the network temporarily splits, until one branch accumulates more work and the other is abandoned. Transactions from the discarded block usually return to the mempool and are confirmed in a later block, so users seldom lose funds, but the miner who produced it loses that block reward and the electricity spent on it. Stale blocks are one reason wallets and exchanges wait for several confirmations before treating a deposit as settled, since very recent blocks can still be replaced.