zenith -
the point above the observer that is directly opposite the nadir on the imaginary sphere against which celestial bodies appear to be projected
celestial point a point in the heavens (on the celestial sphere)
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.
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Ω
zenith Direction pointing vertically above a given location.