Orphan Block
A valid block that loses the race to extend the chain and gets discarded.
An orphan block forms when two miners find a valid block at almost the same moment. The network can only build on one. The chain that gathers the next block wins, and the loser becomes an orphan, its transactions returned to the pool.
Orphans are normal on proof-of-work chains and usually harmless. Miners who produce them earn nothing for the wasted effort. This is one reason exchanges wait for several confirmations before crediting a Bitcoin deposit; a recent block could still be orphaned.
Related terms
A batch of transactions bundled together, time-stamped, and cryptographically linked to the block before it.
ConfirmationEach new block added on top of the one holding your transaction, deepening it and making reversal far harder.
ForkA split in a blockchain's history or rules, whether a brief accident or a deliberate protocol change.