DIN 3750 (August 1957) gives this definition: "Rings with circular cross section made of elastic materials; they seal through the effect of slight bracing during installation, intensified by the operating pressure." The O-ring is also a preferred choice for mechanical seals on account of its advantages compared with other sealing elements:
No risk of faulty installation thanks to the symmetrical cross section, several decades of use, wide application potential, simple calculation and design of connections and dimensions. The O-ring is the sealing element within its operating limits.