Stale Share
A share that arrived at the pool after the block it was working on had already been found.
A stale share is work that was valid when your miner started it but is no longer useful by the time it reaches the pool, because someone else has already found the block. Stales are unavoidable - they happen any time network latency or a slightly slow miner means another participant got there first. A few percent stales is normal. If you are seeing more than that, look at your network connection, distance to the pool, and whether any overclocks are causing your hardware to skip work. Picking a geographically close pool server is the single biggest improvement most home miners can make.