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1(In various mechanisms) a metal ring which secures packing or stuffing that prevents leakage of fluid around a piston rod or other moving part.
2A metal ring forming a tight seal against leakage in the cylinder of a steam engine or internal combustion engine, typically fitted either to the top of the piston or inside the head of the cylinder.
Mid 19th century; earliest use found in Robert Spencer Robinson (1809–1889), naval administrator.