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In general terms, the zenith is the direction pointing directly "above" a particular location (perpendicular, orthogonal). The concept of "above" is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the force of gravity at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir.
The term zenith is also used to represent the highest point reached by a celestial body during its apparent orbit around a given point of observation.[1] 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 impossible for latitudes outside the tropics.
Strictly speaking, the zenith is only approximately contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field.
The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet (for example due to constant displacements of its fluid components) so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).
