An O-ring is a mechanical gasket that is shaped like a torus or doughnut. It is typically made of an elastomer material, such as rubber, that is designed to be compressed between two or more mating surfaces in order to create a seal. O-rings are commonly used in a wide range of applications, including engines, hydraulic systems, and plumbing fixtures, among others. They are known for their reliable sealing capabilities and are widely used in industries that require airtight or watertight seals. O-rings come in a variety of sizes and materials to suit different applications.