Also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus, a donut.
It is a loop of elastomer made to be seated in a groove and compressed during assembly between two or more parts, creating a seal between them.
Used in pressurized stoves.
The Dark Lord didn't need an O-Ring, but his stove did. Come to think of it, he didn't need a stove either. Nevermind.