Zenith

Etymology

From Arabic samt ar-ra’s, "the path over the head," shortened in medieval usage to the element that gave European languages "zenith." Astronomically it is the point directly overhead, opposite the nadir. The word passed through Old Spanish and Medieval Latin, and its figurative sense of a high-water mark followed naturally.

noun · /ZEE-nith/
The highest point of something — the peak of success, power, or intensity.
What it names
The topmost moment or level a thing reaches, from which the only way is level or down; used for careers, empires, and reputations at their height.
Example
At the zenith of her influence, a single memo from her could reshape the entire company’s strategy.
Related idea
Apex is the geometric tip or summit of a thing; zenith carries the astronomical grandeur of the highest point in the sky and suits the peak of a trajectory over time.
noun · /ZEE-nith/
In astronomy, the point in the sky directly overhead an observer.
What it names
the highest point of the celestial sphere above one’s exact location
Example
Near the equator the noon sun climbs almost to the zenith.
Related idea
The peak-of-success sense is this literal overhead point turned metaphor; directly below it lies the nadir.