Definitions for "O-ring"
A type of sealing ring made of a special rubber-like material; in use, the O-ring is compressed into a groove to provide the sealing action.
A round, circular rubber-material gasket shaped like the letter O that is used to seal around round objects, such as the ends of two pipes.
A type of seal. An O-ring is a round elastomeric ring, ideally suited to be a compressed, static seal between non moving parts. O-rings can be used as a face seal on a valve, and used on rotating shafts inside a valve.
a "simple" device often specified or installed as an afterthought - simple that is until it fails