O-rings are solid-rubber seals that are shaped like a doughnut and are round in nature. O-rings block the passage of liquids or gases when pressed between two mating surfaces. O-rings can form a static or dynamic seal. A static seal is where the O-ring does not move and is used simply for containing pressure or maintaining a vacuum. Dynamic seals can be reciprocating (like a piston and cylinder), or rotating (shaft rotating in a housing).
O-Rings are one of the most common and important elements of machine design. Sizes are specified by the inside diameter and the cross-section diameter (thickness).