The position at which a planet or degree of the zodiac crosses the celestial meridian.
The term comes from the Latin 'culminare' (to reach the highest point). It describes the passage of a celestial body or degree of the zodiac over the highest or lowest point on its path from an individual observer's point of view (Medium Coeli, Imum Coeli).
The time from one upper culmination to the next is approximately 24 hours, and between an upper to a lower culmination is approximately 12 hours.
These points are located on the Ecliptic (i.e. below or above the celestial poles, resp.) and not the same as the Zenith and the Nadir.
In most of the northern hemisphere, Polaris, the "North Star," and the rest of the stars of the constellation Ursa Minor can be seen rotating around the celestial pole and are visible at both culminations, as long as the sky is dark enough. Such stars, which never set at the observer's location, are described as circumpolar.