An orphan in blockchain usage is explained in full under orphaned block, which covers why such blocks happen and what becomes of their transactions. This page explains the short noun and how its meaning has shifted.
When two miners on a proof-of-work network find valid blocks at almost the same moment, nodes briefly disagree about which came first. As soon as the next block is built on one of them, that branch becomes the longest chain and the other block is abandoned. Miners and explorers began calling the abandoned block an "orphan", and the miner who produced it receives no block reward.
Strictly, Bitcoin developers once used "orphan" for a different case: a block whose parent a node had not yet received, so it could not be attached to the chain. The abandoned-but-valid block is more precisely called a stale block, and many technical writers now prefer that term. In everyday use, though, "orphan" and "orphaned" usually mean stale. The word should not be confused with an orphan transaction, a transaction that spends an output a node has not yet seen, which waits in the mempool until its parent arrives. Ethereum used the separate terms "uncle" and "ommer" for its own abandoned blocks before moving to proof of stake, and the process of switching branches is described under chain reorganization.