slide valve

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

• noun A valve that slides back and forth over ports, especially one in the cylinder wall of a steam engine that permits the intake and outflow of steam to move the piston.

from The Century Dictionary.

• noun In steam, hydraulic, and pneumatic engineering, a valve which slides over and upon its seat without lifting in opening or closing a port or ports formed in the seat; specifically, a flat-faced plain slide working, or adapted to work or slide, upon a flat-faced seat which includes a port or ports to be alternately opened and closed by the reciprocation of the slide.

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

• noun valve that opens and closes a passageway by sliding over a port