Stale Block
A valid block that doesn't become part of the main chain, often called an 'orphan block' historically.
A stale block is a valid Bitcoin block that didn't end up in the canonical chain. Two miners found blocks at the same height at nearly the same moment; the network split briefly; one side won; the loser is a stale block.
What makes a block stale isn't anything wrong with the block itself - every full node could validate it as correctly formed, with valid signatures, valid proof-of-work, and a valid coinbase claim. It just isn't part of the longest chain anymore. Once a reorg replaces it, the miner who found it gets no block reward; the transactions in the stale block (other than the coinbase) typically end up back in mempools to be mined again later.
Stale blocks are normal. With ~52,000 blocks per year and globally distributed mining, brief block-finding collisions happen several times a year. The longer the average block time and the better block propagation, the lower the stale-block rate. Bitcoin's 10-minute block time was chosen partly to keep stale rates low.
The older term for the same thing is orphan block, which is still sometimes seen in older documentation but has been formally distinguished from a different (rarer) case where a node literally hasn't received the parent block yet.
Stale blocks don't indicate a problem with Bitcoin. They indicate Bitcoin is working - independent miners across the planet racing to extend the chain, with the network converging on whichever side has more work.
Key takeaways
- Lose the race to form the longest chain, invalidating the block reward
- Reflect normal occurrences of block-finding collisions
- Often confused with true orphan blocks (missing known parent block)