What Is a ‘Stale Block’ in the Context of Blockchain Mining?

A stale block is a valid block that was successfully mined but did not make it into the longest, accepted chain of the blockchain. This typically occurs when two or more miners find a block nearly simultaneously, leading to a temporary fork.

The block that is not extended by subsequent miners becomes 'stale' or 'orphaned', and the miner who found it does not receive the block reward. Selfish mining intentionally creates stale blocks for honest miners.