slide valve

noun1

1. •
   A valve in a steam engine, designed to slide back and forth to cover and uncover the openings through which steam enters the cylinder.

   • The inside cylinder, with its piston-valve, had its crank set at 135deg. to the outside-cylinder cranks, which were at the usual 90 deg. to one another; the latter cylinders had slide-valves. — 1961