From Longman Dictionary of Contemporary Englishballcockball‧cock /ˈbɔːlkɒk $ ˈbɒːlkɑːk/ noun [countable] TDHa hollow floating ball on a stick that opens and closes a hole to allow water to flow into a container, for example in a toilet
Examples from the Corpus
ballcock• Automatic top-up with a ballcock will allow all water in a new pool to pass first through the Eradicator.• It had Hayley Syndrome-a missing ballcock.• Meanwhile, I had to go up into the roof and push down the ballcock when things went wrong. 