Capstone is a noun that refers to a crowning achievement.
Our word of the day combines ‘cap’ with ‘stone’ to refer to the final stone in a construction. As a building’s capstone is usually at or near the top, it makes sense that it would evolve into a word for ‘the highest point.’
I feel Gretchen’s last movie was the capstone of her career. It’s hard to imagine her topping that performance.