'Capstone' definitions:

Definition of 'capstone'

(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 'Capstone'

From: GCIDE
  • Capstone \Cap"stone`\, n. (Paleon.) A fossil echinus of the genus Cannulus; -- so called from its supposed resemblance to a cap. [1913 Webster] Capsular