The first definition of horizon in the dictionary is Also called: visible horizon, apparent horizon. the apparent line that divides the earth and the sky. Other definition of horizon is Also called: sensible horizon. the circular intersection with the celestial sphere of the plane tangential to the earth at the position of the observer. Horizon is also Also called: celestial horizon. the great circle on the celestial sphere, the plane of which passes through the centre of the earth and is parallel to the sensible horizon.