1. an imaginary point directly “above” a particular location, on the imaginary celestial sphere. “Above” means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which the gravitational force pulls, is toward the nadir. The term zenith is sometimes used to refer to the highest point reached by a celestial body during its apparent orbit around a given point of observation. The word “zenith” derives from the inaccurate reading of the Arabic expression سمت الرأس (samt ar-ra’s) meaning “direction of the head”/“path above the head”, by Medieval Latin scribes in the Middle Ages (during the 14th century), probably through Old Spanish. It was incorrectly reduced to ‘samt’ (“direction”) and imprecisely written as 'senit’/'cenit’ by those scribes. Through Old French 'cenith’, Middle English 'senith’ and finally 'zenith’ first appears in the 17th century.

2. a highest point or state; culmination; to reach the highest point, summit, or highest development; to end or arrive at a final stage.

Etymology: from Middle English cenyth, from Mediaeval Latin cenit, a transliteration of Arabic سَمْت ‎(samt, “direction, path”) which is in itself a weak abbreviation of سَمْت الرَأْس ‎(samt ar-raʾs, “direction of the head”).

[Joseph Parker - Illumination on the Mountain Top]