Capstone (noun)

1. A stone that forms the top of a wall, building or monument.
2. A crowning achievement, especially one that symbolizes the completion of a project or series of projects.

Origin:

The word 'capstone' is believed to come from the middle english word capstane, which means 'headstone'.

Examples:

1. The capstone of the pyramids was a huge stone block that was carefully placed on top of the structure to symbolize its completion.
2. The capstone of the bridge was finally placed into place, bringing to an end a project that had taken years of hard work and dedication.
3. The capstone of her educational journey was earning her Ph.D. degree and graduating with honors.
4. The capstone of the new park was the construction of a beautiful fountain that was admired by everyone who visited.
5. The capstone of their careers was winning the championship game, a moment that would be remembered for years to come.