The zenith, in Astronomy, is the point in the sky which appears directly above the observer. More precisely, it is the point on the sky with an altitude of +90 degrees, and it is the pole of the Horizontal coordinate system. Geometrically, it is the point on the Celestial sphere intersected by a line drawn from the center of the Earth through your location on the Earth's surface. The point opposite the zenith is the Nadir.
The zenith is, by definition, a point along the local meridian.