a stone placed at the top of a building or wall 拱顶石;压顶石 - (especially North American English)
the best and final thing that somebody achieves, thought of as making their career or life complete (使事业等臻于圆满的)顶点 This award is a fitting capstone to his career. 这个奖项是他职业生涯的一个恰当的顶点。