What Is the Concept of “Stale Blocks” in Mining?

A stale block is a block that was successfully mined by a miner but was not accepted by the rest of the network because another valid block was found and propagated first, thus extending the main chain. The miner's block is essentially an "orphan" that exists outside the longest chain.

Stale blocks represent wasted computational effort and lost revenue for the miner. A high rate of stale blocks can indicate network propagation issues or a high centralization of hash power.