Z is for Zenith

zenith, noun

1. : the point of the sky that is directly above the observer
2. : the highest point reached in the sky by a heavenly body
3. : the best or most successful point or time.

The word developed from an Arabic phrase meaning "the way over one's head".  During the 1300s, English speakers used the term to describe the highest point in the celestial heavens, directly overhead. By the 1600s, zenith was being used for other high points as well.