1. a highest point or state; culmination.
2. the point on the celestial sphere vertically above a given position or observer.
And at the zenith of his fame, how he would suddenly appear at the old village and stalk into church, brown and weather-beaten, in his black velvet doublet and trunks, his great jack-boots, his crimson sash, his belt bristling with horse pistols, his crime-rusted cutlass at his side, his slouch hat with waving plumes, his black flag unfurled, with the skull and crossbones on it, and hear with swelling ecstasy the whisperings, “It’s Tom Sawyer the Pirate!–the Black Avenger of the Spanish Main!” – Mark Twain, The Adventures of Tom Sawyer, 1876
Hence it can hardly be wondered at that as his learning accumulated his practice dissolved, until at the very moment when he had attained the zenith of his celebrity he had also reached the nadir of his fortunes. – Arthur Conan Doyle, The Mystery of Cloomber, 1889
Zenith comes from the Middle English cenith, which in turn traces back to the Old Spanish zenit. The lineage continues to the Arabic samt meaning “road” as in samt ar-rās, “road above one’s head.”