Wiktionary
n. A circular gasket, usually made of rubber.
Wikipedia
An O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.
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, and reliable and have simple mounting requirements. They can seal tens of megapascals (thousands of psi) of pressure.
Usage examples of "o-ring".
It has been determined that defective O-ring seals in Challenger's starboard booster rocket leaked fire to touch off the explosion in the main fuel tank, some nine miles off the ground.
He could see it on her clothes, too, on the flesh pink top with the O-ring zipper that he'd inched down over her breasts.