Zenith means the high point––it comes from astronomy, where it describes the highest point in an arc traveled by a star or a planet or another celestial body. The sun reaches its zenith when it is as high in the sky as it is going to go on that day.
Zenith — and it's opposite, "nadir" — have been appropriated by non-astronomers and are used to describe high and low points in non-astronomical contexts. "The party lasted all night, but the zenith was definitely the moment when we climbed up onto the roof deck and watched the fireworks. It was all downhill from there."
