资料来源 : Webster's Revised Unabridged Dictionary (1913)

Piston ring \Pis"ton ring\ (Mach.)
   A spring packing ring, or any of several such rings, for a
   piston.

资料来源 : WordNet®

piston ring
     n : seal consisting of a split metal ring that seals the gap
         between a piston and the cylinder wall