horizon

(noun) the line at which the sky and Earth appear to meet. Synonyms: apparent horizon, visible horizon, sensible horizon, skyline.

(noun) the range of interest or activity that can be anticipated. Synonyms: view, purview. Example: "It is beyond the horizon of present knowledge."

(noun) a specific layer or stratum of soil or subsoil in a vertical cross section of land.

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