ball cock

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A self-regulating device controlling the supply of water in a tank, cistern, or toilet by means of a float connected to a valve that opens or closes with a change in water level.

Century Dictionary and Cyclopedia

1. n. A hollow sphere or ball of metal attached to the end of a lever, which turns the stop-cock of a water-pipe and regulates the supply of water. The ball, floating in the water of a tank or cistern, rises and falls with it, shutting off the flow when the water has reached a certain level, and letting it on when it falls below this level.

Wiktionary

1. n. alternative spelling of ballcock.

GNU Webster's 1913

1. n. a faucet or valve which is opened or closed by the fall or rise of a ball floating in water at the end of a lever.
2. n. See under Ball.

WordNet 3.0

1. n. floating ball that controls level in a water tank
2. n. floating ball that controls level in a water tank