ball cock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from The Century Dictionary.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Alternative spelling of ballcock.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun floating ball that controls level in a water tank
• noun floating ball that controls level in a water tank