ho·ri·zon 
(hə-rīzən)
(hə-rīzən)n.
1. The apparent intersection of the earth and sky as seen by an observer. Also called apparent horizon.
2. Astronomy
a. See sensible horizon.
b. See celestial horizon.
c. The limit or edge of the observable universe.
3. The range of one's knowledge, experience, or interest.
4. Geology
a. A specific position in a column of rock layers, usually designated by the occurrence of one or more distinctive fossils or by a distinctive sediment bed, that is used in stratigraphy.
b. A layer of soil that can be distinguished from adjacent layers of soil and that is characterized by a certain color, texture, structure or chemical composition.
5. 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.
[Middle English orizon, from Old French, from Latin horizōn, from Greek horizōn (kuklos), limiting (circle), horizon, present participle of horizein, to limit, from horos, boundary.]