stop valve

Century Dictionary and Cyclopedia

1. n. In hydraulics, a valve which closes a pipe against the passage of fluid. It is usually a disk which occupies a chamber above the pipe when the passageway through the latter is open, and is driven down by a screw to stop the aperture.
2. n. 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.

Wiktionary

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

GNU Webster's 1913

1. n. a valve that can be closed or opened at will, as by hand, for preventing or regulating flow, as of a liquid in a pipe; -- in distinction from a valve which is operated by the action of the fluid it restrains.