![piston ring piston ring](https://www.tekportal.net/wp-content/uploads/2018/11/piston-ring.jpg)
noun
- 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.
noun
- 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