Definitions for "Prime vertical"
Great circle that crosses the meridian of the birth place at right angles.
The vertical East-West great circle, whose plane is perpendicular to the plane of the meridian, and which passes through the East Point, Zenith, West Point, and Nadir of a particular location. An observer who is standing upright and facing south will be standing in the plane of the prime vertical.
A great circle on the celestial sphere that passes through the zenith. It is a vertical circle that is bisected by the meridian. Degrees run from east to west.