An O-ring is a circular rubber gasket or sealing ring with a round cross-section, typically made from elastomers such as rubber or silicone. The name "O-ring" comes from its shape, which resembles the letter "O" when viewed in cross-section. These rings are widely used in various industries for sealing applications to prevent the leakage of liquids or gases.

Key features of O-rings include their flexibility, resilience, and ability to form a tight seal when compressed between two mating surfaces. They are commonly used in hydraulic and pneumatic systems, as well as in engines, pumps, valves, and other mechanical systems. O-rings are known for their simplicity and effectiveness in creating a reliable seal.