a finishing stone of a structure.
the crowning achievement, point, element, or event.

1350–1400; Middle English. See cap1, stone Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
capstone or copestone (ˈkæpˌstəʊn, ˈkəʊpˌstəʊn)
1.  one of a set of slabs on the top of a wall, building, etc
2.  mountaineering a chockstone occurring at the top of a gully or chimney
3.  a crowning achievement; peak: the capstone of his career
copestone or copestone

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

topmost stone in a construction, 1685, from cap + stone. Earliest use is figurative.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Yet neither of those turnarounds could compare to the evening's
  you-have-got-to-be-kidding-me capstone.
Now comes the book he hopes will be the capstone of his career.
The capstone was the colossal new train station to be built in midtown.
Graduation is a capstone ceremony to celebrate the work of the matriculants in
  this centuries-old system.
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