horizon (həˈraɪz^{ə}n)  
—n  
1.  visible horizon, Also called: apparent horizon the apparent line that divides the earth and the sky 
2.  astronomy 
a. Also called: sensible horizon the circular intersection with the celestial sphere of the plane tangential to the earth at the position of the observer  
b. 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  
3.  the range or limit of scope, interest, knowledge, etc 
4.  a thin layer of rock within a stratum that has a distinct composition, esp of fossils, by which the stratum may be dated 
5.  A horizon B horizon See C horizon a layer in a soil profile having particular characteristics 
6.  on the horizon likely or about to happen or appear 
[C14: from Latin, from Greek horizōn kuklos limiting circle, from horizein to limit, from horos limit]  
ho'rizonless  
—adj 
celestial horizon
A great circle on the celestial sphere having a plane that passes through the center of the Earth at a right angle to the line formed by an observer's zenith and nadir. The celestial horizon divides the celestial sphere into two equal hemispheres based on the observer's location, with one hemisphere representing the half of the sky visible to the observer at that location and the other representing the half that is hidden from the observer below the Earth's horizon. The celestial horizon is used as the reference point in determining a celestial body's altitude. Also called rational horizon. Compare sensible horizon. 
horizon (hərī'zən) Pronunciation Key
