horizon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The apparent intersection of the earth and sky as seen by an observer.
• noun The limit or edge of the observable universe.
• noun The range of one's knowledge, experience, or interest.
• noun 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.
• noun 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.
• noun Archaeology A period during which the influence of a specified culture spread rapidly over a defined area.

from The Century Dictionary.

• To limit or bound by a horizon.
• noun The circle which at sea forms the apparent boundary between sea and sky, and on land would bound the sky were all terrestrial obstructions down to the sea-level removed. Called the apparent, sensible, or visible horizon, in distinction from the astronomical horizon (which see, below).
• noun Hence The line that bounds the view; the limit of vision.
• noun Figuratively, the limit of intellectual perception, of experience, or of knowledge.
• noun In geology, a stratum or group of strata characterized by the presence of a particular fossil not found in the underlying or overlying beds, or of a peculiar assemblage of fossils.
• noun In zoology and anatomy, a level or horizontal line or surface: as, the horizon of the teeth; the horizon of the diaphragm

from the GNU version of the Collaborative International Dictionary of English.

• noun The line which bounds that part of the earth's surface visible to a spectator from a given point; the apparent junction of the earth and sky.
• noun A plane passing through the eye of the spectator and at right angles to the vertical at a given place; a plane tangent to the earth's surface at that place; called distinctively the sensible horizon.
• noun A plane parallel to the sensible horizon of a place, and passing through the earth's center; -- called also rational horizon or celestial horizon.
• noun (Naut.) The unbroken line separating sky and water, as seen by an eye at a given elevation, no land being visible.
• noun (Geol.) The epoch or time during which a deposit was made.
• noun (Painting) The chief horizontal line in a picture of any sort, which determines in the picture the height of the eye of the spectator; in an extended landscape, the representation of the natural horizon corresponds with this line.
• noun The limit of a person's range of perception, capabilities, or experience.
• noun A boundary point or line, or a time point, beyond which new knowledge or experiences may be found.
• noun See under Apparent.
• noun a level mirror, as the surface of mercury in a shallow vessel, or a plane reflector adjusted to the true level artificially; -- used chiefly with the sextant for observing the double altitude of a celestial body.
• noun (Astron.) See def. 2, above.
• noun (Astron.) the vertical angle between the sensible horizon and a line to the visible horizon, the latter always being below the former.
• noun (Astron.) See def. 2, above.
• noun See definitions 1 and 2, above.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The horizontal line that appears to separate the Earth from the sky.
• noun The range or limit of one's knowledge, experience or interest.
• noun geology A specific layer of soil or strata
• noun archaeology, US A cultural sub-period or level within a more encompassing time period.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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
• noun the line at which the sky and Earth appear to meet
• noun the range of interest or activity that can be anticipated