An O'ring is a mechanical gasket in the shape of a circle; it is a loop of pliable material with a disc-shaped cross-section, designed to be seated into a groove and compressed during assembly between two or more parts, creating a seal.
The O'ring may be used in static applications or in dynamic applications where there is relative motion between the parts and the O'ring. Dynamic examples include rotating pump shafts and hydraulic cylinder pistons.
O'rings are one of the most common seals used in machine design because they are inexpensive, easy to make, reliable, and have simple mounting requirements. They can seal tens of thousands of pounds per square inch of pressure.