The poles of the celestial horizon are known as the zenith and the nadir.
Let an observer $O$ on the surface of Earth be assumed to be the center of the celestial sphere $C$.
The zenith $Z$ is the point on $C$ which is vertically overhead.
Consider the celestial sphere $C$ with an observer $O$ at the center.
Let $Z$ be the zenith of $C$
Let the straight line $ZO$ be produced from $O$ to meet $C$ at $R$.
The point $R$ is known as the nadir of $C$.