horizon in WordNet English dictionary

1. the line at which the sky and Earth appear to meet
   horizon  apparent horizon  visible horizon  sensible horizon  skyline 
   
2. the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth
   horizon  celestial horizon 
   
3. the range of interest or activity that can be anticipated
   "It is beyond the horizon of present knowledge"
   horizon  view  purview 
   
4. a specific layer or stratum of soil or subsoil in a vertical cross section of land
   horizon