A circle in the sky, running from the zenith, straight down, and perpendicular to the horizon -- and therefore, vertical with respect to the observer.

a plane perpendicular to the local horizon and passing through the center of the Earth. The "Meridian" is a vertical circle passing through the north and south points on the horizon; whereas the "Prime Vertical" is a vertical circle that passes through the east and west points on the horizon.

a great circle on the celestial sphere passing through the zenith and perpendicular to the horizon

Great circle passing through the observer's zenith and nadir.

In astronomy, a vertical circle is a great circle on the celestial sphere that is perpendicular to the horizon. Therefore it passes through the zenith and the nadir. There is a vertical circle for any given direction.