The zenith is the point in the sky vertically straight above the observer.
It is the point where the observer's sight-line is perpendicular to the Earth's atmosphere, and hence where the smallest amount of air – termed the airmass – lies along it. For this reason, objects close to the zenith are subject to less atmospheric distortion than those anywhere else on the sky, and astronomers usually try to observe objects when they are as high in the sky as possible.
Specifically, an object makes its closest approach to the zenith at the moment when it culminates. However, some designs of telescope, and most designs of binoculars, are very awkward to use close to the zenith, meaning that a compromise must be reached.