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.

Dates when the sun is at an observer's zenith

Vernal Equinox - around March 21 - Equator
Summer Solstice - around June 21 - 23.5 degrees North
Autumnal Equinox - around September 23 - Equator
Winter Solstice - around December 21 - 23.5 degrees South

"Zenith" also means "The Highest Point" so you can use it in a phrase like: The Zenith of land in the world is Mount Everest.