Definition
An O-ring is a doughnut-shaped object, or torus. The opposite sides of an O-ring are squeezed between the walls of the cavity or "gland" into which the O-ring is installed. The resulting zero clearance within the gland provides an effective seal, blocking the flow of liquids or gases through the gland's internal passage.
An O-ring is defined by its dimensions (based on inside [hole] diameter and cross section), durometer (Shore A hardness), and material composition.