In general terms, the zenith is the direction pointing directly above a particular location. The concept of above is more specifically defined in astronomy, geophysics and related sciences as the vertical direction opposite to the force of gravity at a given location. The opposite direction, 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. This sense of the word is 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 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).
Nadir
The nadir is the direction pointing directly below a particular location (perpendicular, orthogonal). Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite of the nadir is the zenith.
Nadir also refers to a downward-facing viewing angle of an orbiting satellite, such as is employed during remote sensing of the atmosphere, as well as when an astronaut faces the Earth while performing an EVA.
Extra-vehicular activity (EVA) is work done by an astronaut away from the Earth, and outside of a spacecraft.
