O RING 1 : "O" ring is a doughnut-shaped ring of rubber used as a seal around the periphery of the mating insulator interface of cylindrical connectors
US Department of Labor, Electric Power eTool: Glossary of Terms, Occupational Safety & Health Administration OSHA, Washington, D.C., USA 2010. https://www.osha.gov/SLTC/etools/electric_power/gl ossary.html, (OSHA/ Electr Pwr - Amer) {618}