Middle English cenyth, senyth, from Middle French cenit, from Medieval Latin, from Old Spanish zenit, modification of Arabic samt (al-ra's) way (over one's head)
The word "zenith" derives from the inaccurate reading of the Arabic expression سمت الرأس (samt ar-ra's), meaning "direction of the head" or "path above the head", by Medieval Latin scribes in the Middle Ages (during the 14th century), possibly through Old Spanish. It was reduced to 'samt' ("direction") and miswritten as 'senit'/'cenit', as the "m" was misread as an "ni". Through the Old French 'cenith', 'zenith' first appeared in the 17th century.
- 1: the point of the celestial sphere that is directly opposite the nadir and vertically above the observer — see azimuth illustration
- 2: the highest point reached in the heavens by a celestial body
- 3: culminating point : acme <at the zenith of his powers — John Buchan>
The zenith is 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 gravity 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. This sense of the word is often used to describe the location of the Sun ("The sun reached its zenith..."), but to an astronomer the sun doesn't have its own zenith, and is at the zenith only if it is directly overhead.
In astronomy, the altitude in the horizontal coordinate system and the zenith angle are complementary angles, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the celestial sphere that passes through the zenith, nadir, and the celestial poles.
Zenith is also used to determine the time of Dhuhr (midday) prayer by Muslims which is when the sun passes the zenith.