When a Bitcoin block is successfully hashed any other attempt to hash it is futile because it is now “stale”. Subsequent attempts would simply be repeating work that has been completed for no reward. When a block is “discovered” any other miners working on solving the block stop and move on to the next one. The block they were working on now becomes “stale”. The term is also used in mining pools to describe a share of a hashing job that has already been completed.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc