capstone

English[edit]

From Middle English capston; equivalent to cap +‎ stone.

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.

any of the stones making up the top layer of a wall

a crowning achievement