an o-ring is a flat, usually circular piece of plastic or rubber that is used in many machines, equipment, most often where some sort of a seal is necessary.

the o-ring works as a gasket, it prevents liquid or gas from escaping.

when an o-ring is damaged or worn down, it must be replaced, which generally requires temporary dismantling. the o-ring seal can be extremely important, and thusly it is necessary to check on them from time to time.

let me relate to you a little tale of what happens when o-rings are not checked regularly: i've been known to work, off and on, with my father in the insulating business. polyurethane spray-foam insulation, that is. the machines for said purpose have o-rings in various locations, and it just so happens that not all too long ago, one o-ring (which was in need of replacement) got to the point where it couldn't handle the stress, and it completely blew away from the pressure. this resulted in a terribly exciting chain reaction of events, ending with a lot of chemical being spewed out in a fountainesque manner. bad stuff.

o-rings come in varying sizes, and make nifty engagement bands.