O-ring
O-ring turns up in 2 fields: vacuum, mechanical. Here is what it means to the people in each one.
O-ring in vacuum
Elastomer ring with a round cross-section that seals demountable vacuum joints by being squeezed into place.
A round-section elastomer ring compressed between flange faces, in a groove or on a centring ring. Viton (FKM) is the default for vacuum because it outgasses and permeates less than nitrile and takes bakes to about 150 °C; nitrile is cheaper for rough service, and perfluoroelastomers handle aggressive process chemistry. Seals go in clean and dry, or with at most a thin film of vacuum grease, and because they pass helium and water they limit elastomer-sealed systems to high vacuum.
"Wipe the O-ring with a lint-free wipe, no grease, and check it for cuts before you clamp the lid."
O-ring in mechanical
A circular elastomer gasket with a round cross-section used to seal fluid or gas connections.
Seated in a machined groove and compressed between two or more parts, an O-ring creates a tight seal at the interface. As fluid pressure increases, it deforms the elastomer against the groove walls, actually improving the seal up to a certain failure point. They are the most common and reliable seals in hydraulic cylinders, pipe flanges, and engine components.
"Make sure you lubricate the O-ring before sliding the piston in, or it will pinch and leak."