the point above the observer that is directly opposite the nadir on the imaginary sphere against which celestial bodies appear to be projected
"At local noon, the sun reached its zenith, appearing directly overhead as it stood opposite the nadir."
The point in the sky vertically above a given position or observer; the point in the celestial sphere opposite the nadir.
"The pilot adjusted his instruments to navigate directly toward the aircraft's zenith, where the sun stood perfectly overhead."
In plain English: Zenith is the highest point something can reach, like when you are at your absolute best or most successful moment.
"The band's popularity reached its zenith when their latest album topped the charts worldwide."
Usage: Use "zenith" to denote the peak of success, power, or achievement rather than its literal astronomical meaning when discussing abstract concepts. Avoid confusing it with "nadir," which signifies the lowest point, as they represent opposite extremes on a vertical scale.
The word "zenith" comes from the Arabic phrase samt ar-raʔs, meaning "direction of the head." It entered English via Medieval Latin and originally referred to the point in the sky directly above an observer.