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Zenith word has following meanings:

The point on the celestial sphere that is directly above the observer
The upper region of the sky
The highest point above the observer's horizon attained by a celestial body
The point of culmination, the peak.

In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenith also refers 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, but it is only technically accurate for one latitude at a time and only possible at the low latitudes.

The zenith is used in the following scientific contexts:

It serves as the direction of reference for measuring the zenith angle, which is the angular distance between a direction of a star and the local zenith, relative to the point for which the zenith is defined.
It defines one of the axes of the horizontal coordinate system in astronomy.