An orphan block is a valid block that does not join the main blockchain because its parent block is missing or another chain becomes longer. Simply put, it is a block that looks correct but is not used by most nodes.
Blocks connect by referencing the previous block. Sometimes, two miners create different valid blocks from the same parent at almost the same time. This leads to two versions of the chain for a short period. Nodes usually keep building on the chain they saw first, and miners may continue on separate branches. As more blocks are added, one chain grows longer and the shorter one is dropped. The blocks from the dropped branch become orphan blocks. Delays in the network, differences in software, and the distance between nodes can make this more likely.
Transactions in orphan blocks are not lost. When a block is orphaned, many of its transactions go back to the pool of unconfirmed transactions, called the mempool. Miners and nodes can then add these transactions to new blocks on the main chain. If a transaction from an orphan block is already confirmed by another block, it is not in the mempool anymore.
Writers sometimes use these terms differently. Some call unused blocks on the shorter branch orphan blocks, while others use the term stale block. Both mean a valid block that is not part of the main chain. The choice depends on context and how the protocol’s software has used the terms in the past.
If a miner creates an orphan block, the block’s coinbase reward and transaction fees are not paid out on the main chain. This means the miner does not get the expected reward unless a later reorganization adds that block to the main chain. Mining pools and individual miners handle this risk by changing their strategies and sharing rewards within pools to even out results.
Modern node software helps reduce orphan blocks by checking parent references before accepting a block and by improving how blocks are shared. Faster sharing methods, relay networks, and better ways to handle missing parent information make it less likely for a valid block to become permanently orphaned. However, short-lived forks still happen in proof of work systems when several parties try to add blocks at once.
Orphan blocks can be used in some attack strategies. An attacker with enough hashing power might secretly mine an alternate chain. If the attacker later releases a longer chain, nodes will switch to it, and blocks from the old chain could become orphaned. This is linked to attacks like double spends or 51 percent attacks. Orphaned blocks can also show up when someone tries to manipulate how blocks are shared or how transactions are ordered.
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