(n.) The circle 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.
(n.) 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.
(n.) A plane parallel to the sensible horizon of a place, and passing through the earth's center; -- called also rational / celestial horizon.
(n.) The unbroken line separating sky and water, as seen by an eye at a given elevation, no land being visible.
(n.) The epoch or time during which a deposit was made.
(n.) 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.