horizon horizon /hɝˈaɪzən/

noun

1. () The line at which the sky and Earth appear to meet  ( apparent horizon , sensible horizon , skyline , horizon , visible horizon )
   the line at which the sky and Earth appear to meet
     They watched the sun set below the horizon, painting the sky in shades of purple and gold.
   They watched the sun set below the horizon, painting the sky in shades of purple and gold.
   
   
2. () The range of interest or activity that can be anticipated  ( purview , view , horizon )
   the range of interest or activity that can be anticipated
    It is beyond the horizon of present knowledge
   It is beyond the horizon of present knowledge
   
3. The great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth  ( celestial horizon , horizon )
   the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth
   
4. A specific layer or stratum of soil or subsoil in a vertical cross section of land  ( horizon )
   a specific layer or stratum of soil or subsoil in a vertical cross section of land