noun (n)

• the line at which the sky and Earth appear to meet(noun.location)

  Synonym:
  apparent horizon, sensible horizon, skyline, visible horizon
  

  source: wordnet30
• the range of interest or activity that can be anticipated(noun.attribute)

  Example:
  It is beyond the horizon of present knowledge.
  

  Synonym:
  purview, view
  

  source: wordnet30
• a specific layer or stratum of soil or subsoil in a vertical cross section of land(noun.location)

  source: wordnet30
• the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth(noun.location)

  Synonym:
  celestial horizon
  

  source: wordnet30
• The circle which bounds that part of the earth's surface visible to a spectator from a given point; the apparent junction of the earth and sky.(noun)

  source: webster1913