Crypto glossary
Orphan block (stale block)
An orphan or stale block is a valid block that ends up outside the main chain because a competing block at the same height was accepted by the network instead.
Two miners sometimes find a valid block at almost the same moment. Part of the network builds on one, part on the other, and the tie breaks when the next block extends one branch. The losing block is discarded: its transactions return to the mempool and its miner loses the reward. Strictly, Bitcoin developers call these stale blocks, reserving orphan for blocks whose parent is unknown, but both terms are used interchangeably in practice.
Stale rates reveal how well blocks propagate. Faster relay techniques such as compact blocks have made them rare on Bitcoin. Ethereum's proof-of-work era had frequent competing blocks and partly rewarded them as uncles; that mechanism disappeared with the move to proof of stake.
For users, an orphaned block is the reason one confirmation is not final. A payment in a block that becomes stale is not lost, it simply waits to be mined again, but if a conflicting transaction is confirmed first, the original can be invalidated.
Example
Two pools find block 900,001 seconds apart; the next block is built on the first pool's version, so the second pool's block becomes stale and its 3.125 BTC subsidy is never paid.