Zenith

In astronomy, the zenith is a point "above" a particular location, in the sky. "Above" means in the opposite direction of gravity at that location. The zenith is the opposite of the nadir. The nadir is in the direction the direction that gravity pulls. The zenith is not a real point, it is used as a reference for the location of other things in the sky. 
Relevance and use.
For scientists, the zenith is the direction of reference for measuring the zenith angle. The zenith angle is the angle between a place in the sky (usually a star) and the local zenith.