A stale block (stale) is the result of a miner's computations that turned out to be out of date by the time it was submitted to the network. The situation arises when a miner spends power on a block that has already been found and accepted by another network participant. Since the blockchain accepts only one valid chain, re-hashing an already confirmed block becomes useless work and is not counted by the network.
In the context of mining pools the term is used a little more broadly. Here stale refers to a "share" — a portion of completed work that arrived at the pool late, after another miner had already found a solution for the current block. Such stale shares are usually not paid or are paid at a reduced rate, which affects the participant's overall income.
A high proportion of stale blocks and shares often indicates technical problems: a large network delay (ping) to the pool server, an unstable connection or suboptimal equipment settings. Reducing delays and choosing a geographically close pool help cut the number of stale results and increase mining efficiency.
