The point at which a perpendicular from any given location on the Earth's surface intersects the Celestial Sphere. This point is located directly above the observer.
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.
Although in poetical language it is possible to say "the Sun stood at its zenith", this is not actually (physically) possible in the middle latitudes.
The nadir is the point opposite the zenith.
In astrology, the zenith often is mistakenly thought to be equivalent to the Medium Coeli.
Notes and References
- "Middle latitudes" regarding Europe and North America; it's the other way round in Australia and South America