1. The apparent intersection of the earth and sky as seen by an observer. Also called apparent horizon.
2. Astronomy.
   1. The sensible horizon.
   2. The celestial horizon.
   3. The limit of the theoretically possible universe.
3. The range of one's knowledge, experience, or interest.
4. Geology.
   1. 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.
   2. A specific layer of soil or subsoil in a vertical cross section of land.
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, from Greek horizōn (kuklos), limiting (circle), horizon, present participle of horizein, to limit, from horos, boundary.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.