One axis of the altazimuth coordinate system. It is the circle on the celestial sphere midway between the observer's zenith and nadir. In laymen's terms, if you were standing in the center of a globe, the north pole would be your zenith, the south pole would be your nadir, and the equator would be your celestial horizon

An observer on the surface of the earth may be able to see points below his celestial horizon due to atmopsheric refraction.