'Copestone' definitions:

Definition of 'copestone'

(from WordNet)
noun
A final touch; a crowning achievement; a culmination [syn: finishing touch, capstone, copestone]
noun
A stone that forms the top of wall or building [syn: capstone, copestone, coping stone, stretcher]

Definition of 'Copestone'

From: GCIDE
  • Copestone \Cope"stone`\, n. (Arch.) A stone for coping. See Coping. [1913 Webster]