The word zenith has two meanings: 1. the time at which something is most powerful or successful.  E.g. "under Justinian, the Byzantine Empire reached its zenith of influence" and 2. the point in the sky or celestial sphere directly above an observer or the highest point reached by a celestial or other object.  E.g. "the sun was well past the zenith"

When you reach the zenith, you're at the top, the pinnacle, the summit, the peak.  Zenith developed from an Arabic phrase, samt ar-ra's, meaning "the way over one's head," and then traveled through Old Spanish, Medieval Latin (cenit), and Middle French before arriving in English. 

According to the Oxford English Dictionary (OED), the word "zenith" was first used in English during the Middle English period, before 1387, in a translation by John Trevisa. 

Examples of the word "zenith" used in a sentence:  
"It may be in the fall, though, that this place reaches its zenith." 
"The summer sun was at its zenith in a cloudless sky." 
"The sun is well past its zenith when we cross paths with a mini-flotilla." 
"His publicity campaign reached its zenith when he was summoned to an audience with the royal family."  
"The tragedy will no doubt reach its terrible zenith in next week's final episode."
"The second world war represented the zenith of her career, but also led to her inexorable decline." 
"The power of the archdeacon reached its zenith at the outset of the 13th century." 
"The exhibitions in the arena were perhaps at their zenith during his tenure of power." 

Just the facts:

ZENITH Definition & Meaning - Merriam-Webster

ZENITH Synonyms: 47 Similar and Opposite Words | Merriam-Webster Thesaurus