Definify.com

Webster 1913 Edition

Noun.

(Paleon.)

A fossil echinus of the genus

Cannulus

; – so called from its supposed resemblance to a cap.

capstone ‎(plural capstones)

Any of the stones making up the top layer of a wall; a coping stone.
(figuratively) A crowning achievement, culmination or finishing touch.
- 1904, Guy Wetmore Carryl, Far from the Maddening Girls, chapter 5
  “You see, I’ve never had a girl friend,” I added, by way of topping the obelisk of silliness with the capstone of fatuity.
- 1969, NASA, The Post-Apollo Space Program: Directions for the Future
  Success of the Apollo program has been the capstone to a series of significant accomplishments for the United States in space in a broad spectrum of manned and unmanned exploration missions and in the application of space techniques for the benefit of man.

a crowning achievement