What Is a ‘Stale Share’ and Why Does It Reduce a Miner’s Reward?

A stale share is a valid share submitted by a miner to the pool that arrives after the pool has already found a block or after a new block has been broadcast by the network. The work is considered wasted because it was based on an outdated block header.

Stale shares reduce a miner's reward because they are typically not counted as valid contributions toward the reward calculation.