stop valve

from The Century Dictionary.

• noun In hydraulics, a valve which closes a pipe against the passage of fluid.
• noun In steam-engines, a valve fitted to the steam-pipes, where they leave the several boilers, in such a way that any boiler may be shut off from the others and from the engines.

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

• noun A valve that may be turned to regulate the flow of fluid through a pipe (especially water through a mains supply)