piston ring

WordReference Random House Unabridged Dictionary of American English © 2025

pis′ton ring′, 

1. Automotive, Mechanical Engineering a metallic ring, usually one of a series, and split so as to be expansible, placed around a piston in order to maintain a tight fit, as inside the cylinder of an engine. Also called packing ring, ring. 

• 1865–70

Collins Concise English Dictionary © HarperCollins Publishers::

piston ring noun

1. a split ring, usually made of cast iron, that fits into a groove on the rim of a piston to provide a spring-loaded seal against the cylinder wall