The Collaborative International Dictionary
Snifting valve
Snifting \Snift"ing\, a. & n. from Snift.
Snifting valve, a small valve opening into the atmosphere from the cylinder or condenser of a steam engine, to allow the escape of air when the piston makes a stroke; -- so called from the noise made by its action.
Wiktionary
snifting valve
n. A small valve opening into the atmosphere from the cylinder or condenser of a steam engine, to allow the escape of air when the piston makes a stroke.
Wikipedia
Snifting valve
A snifting valve (sometimes snifter valve) is an automatic anti-vacuum valve used in a steam locomotive when coasting. The word Snift imitates the sound made by the valve.