poppet valve

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

• noun An intake or exhaust valve, operated by springs and cams, that plugs and unplugs its opening by axial motion.

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

• noun mechanics A valve (as in an internal combustion engine) which consists of a sliding shaft with a disk on the end (the disk being shaped to alternately plug and unplug a matching port) and which is operated by springs and possibly cams interacting with the shaft.

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

• noun a mushroom-shaped valve that rises perpendicularly from its seat; commonly used in internal-combustion engines