What is the meaning of horizon?

noun

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