Definitions:

1. A ring-shaped seal with a round cross-section, similar to a donut, that is typically made of rubber. NOUN
   • The failure of an O-ring caused the explosion that destroyed the space shuttle Challenger in 1986.
2. A rubber ring used in machinery as a seal against oil, air, etc. NOUN
   • O-rings are cheap, ease to produce, reliable, and easy to fit.

Synonyms:

• annular gasket
• ring seal
• sealing ring
• sealing washer