It's a mechanical gasket in the shape of a torus; a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between parts, creating a seal at the interface.
It was an o-ring failure that caused the loss of the Space Shuttle Challenger and its crew - so take care.
A specialist camera shop will be able to advise you where to get one.
Peccavi, July 2009
