Source : Webster's Revised Unabridged Dictionary (1913)

Copestone \Cope"stone`\, n. (Arch.)
   A stone for coping. See {Coping}.

Source : WordNet®

copestone
     n 1: a final touch; a crowning achievement; a culmination [syn: {finishing
          touch}, {capstone}]
     2: a stone that forms the top of wall or building [syn: {capstone},
         {coping stone}, {stretcher}]