Orphan

An orphan block forms when two miners, or on proof-of-stake networks two validators, produce a valid block for the same position in the chain at nearly the same moment. The network briefly holds two competing candidates, and once nodes agree on which branch to extend, whichever chain carries the most accumulated work (or wins the fork-choice rule under proof-of-stake) becomes canonical. The losing block drops out of the chain permanently and never earns confirmations.

This happens because block propagation across a peer-to-peer network is never instantaneous. A miner on the far side of the world can find a valid block seconds before hearing that a rival already solved the same height. Chains with short block times or slower propagation across their node network are more prone to producing these near-simultaneous blocks.

Transactions that were confined only to the orphaned block are not lost; they return to the mempool and are usually picked up again in a later block, though this can briefly delay settlement. The block reward and fees the losing miner would have earned are forfeited, which is one reason exchanges and payment processors wait for multiple confirmations before treating a transaction as final, especially for large transfers.

Ethereum's old proof-of-work chain handled the same collision differently: instead of discarding these blocks outright, it partially rewarded them as uncle blocks. That mechanism vanished once Ethereum moved to proof-of-stake in 2022, since validators no longer race to solve a puzzle. On Bitcoin, true orphans, blocks whose parent is unknown to a node, are now rare thanks to headers-first propagation adopted in 2015; most blocks popularly called "orphaned" today are more precisely stale blocks.