See horizon in Webster's New World College Dictionary
noun
the line where the sky seems to meet the earth
the limit or extent of one's outlook, experience, interest, knowledge, etc.: travel broadens one's horizons
an archaeological level or an area of culture as indicated by surviving artifacts
Astron. the great circle on the celestial sphere perpendicular to the line from the observer's zenith to the nadir
Geol. a layer of soil or rock identified by physical characteristics, particular fossils, etc.
See horizon in American Heritage Dictionary 4
(hə-rīˈzən)
noun
The apparent intersection of the earth and sky as seen by an observer. Also called apparent horizon.
Astronomy
a. The sensible horizon.
b. The celestial horizon.
c. The limit of the theoretically possible universe.
The range of one's knowledge, experience, or interest.
Geology
a. A specific position in a stratigraphic column, such as the location of one or more fossils, that serves to identify the stratum with a particular period.
b. A specific layer of soil or subsoil in a vertical cross section of land.
Archaeology A period during which the influence of a specified culture spread rapidly over a defined area: artifacts associated with the Olmec horizon in Mesoamerica.