Definitions of horizon:

• noun:   the range of interest or activity that can be anticipated

  Example: "It is beyond the horizon of present knowledge"

• noun:   the line at which the sky and Earth appear to meet
• noun:   the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth
• noun:   a specific layer or stratum of soil or subsoil in a vertical cross section of land