The
zenith is the point in the sky where you are looking when you look 'straight up' from the ground. More precisely, it is the point on the sky with an
altitude of +90 Degrees; 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 zenith is, by definition, a point along the local meridian.
This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml