An O-ring, also known as a packing, or a toric joint, is a mechanicalgasketin the shape of atorus; it is a loopelastomerwith a roundcross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating asealat 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 rotatingpumpshafts andhydraulic cylinderpistons.

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 tensmegapascals(thousandspsi) of pressure.