piston ring

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

• noun An adjustable split metal ring that fits around a piston and seals the gap between the piston and the cylinder wall.

from The Century Dictionary.

• noun A packing-ring; a ring, usually of cast-iron, brass, or steel, placed in a groove in a piston for the purpose of preventing the steam or gas from passing the piston.

from the GNU version of the Collaborative International Dictionary of English.

• (Mach.) A spring packing ring, or any of several such rings, for a piston.

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

• noun A ring or seal that fits around a piston, sealing between the piston and the bore in which it slides.

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

• noun seal consisting of a split metal ring that seals the gap between a piston and the cylinder wall