orphan block
A valid block that was not included in the main Bitcoin blockchain because another block at the same height was accepted first.
An orphan block is a valid block that loses the competition to become part of the main chain. This happens when two miners find a valid block at nearly the same time and broadcast it to the network. Nodes begin building on whichever block they receive first, creating a temporary fork. When the next block is found on one of the branches, that branch becomes longer. The entire network then abandons the shorter branch and adopts the longer one, leaving the block on the shorter branch as an orphan.
The miner who produced the orphan block receives no block reward and no transaction fees, even though the block itself was technically valid and required real computational work. This is simply how proof-of-work consensus resolves competing valid blocks: only the chain with the most accumulated work survives. Orphan blocks are a normal, occasional occurrence in Bitcoin and do not indicate any attack or malfunction.
From the network's perspective, orphan blocks are harmless. Transactions in an orphaned block that were not included in the winning block return to the mempool and are picked up by later miners. The network continues without any lasting disruption. The term is sometimes confused with uncle blocks in Ethereum, which have a different mechanism, or with invalid blocks rejected for rule violations. An orphan block is distinct from both: it followed all rules but simply arrived too late.