Alternative term: Rational horizon
The great circle on the celestial sphere, every point of which is 90 degrees from the observer's zenith. This circle encloses the horizontal plane through the observer and divides the celestial sphere into two hemispheres.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM