Most commonly, the horizon is the border between the surface of a celestial body and its sky when viewed from the perspective of an observer on or above the surface of the celestial body. This concept is further refined as -

• The true or geometric horizon, which an observer would see if there was no alteration from refraction or from obstruction by intervening objects. The geometric horizon assumes a spherical earth. The true horizon takes into account the fact that the earth is an irregular ellipsoid. When refraction is minimal, the visible sea or ocean horizon is the closest an observer can get to seeing the true horizon.

• The refracted or apparent horizon, which is the true horizon viewed through atmospheric refraction. Refraction can make distant objects seem higher or, less often, lower than they actually are. An unusually large refraction may cause a distant object to appear ("loom") above the refracted horizon or disappear ("sink") below it.

• The visible horizon, which is the refracted horizon obscured by terrain and, on Earth, by life forms such as trees and/or human constructs such as buildings.

There is also an imaginary astronomical, celestial, or theoretical horizon, part of the horizontal coordinate system, which is an infinite eye-level plane perpendicular to a line that runs (a) from the center of a celestial body (b) through the observer and (c) out to space (see graphic above). It is used to calculate "horizon dip," which is the difference between the astronomical horizon and the sea horizon measured in arcs. Horizon dip is one factor taken into account in navigation by the stars.

In perspective drawing, the horizon line (also referred to as "eye-level") is the point of view from which the drawn scene is presented. It is an imaginary horizontal line across the scene. The line may be above, level with, or below the center of the drawing, corresponding to looking down, straight at, or up to the drawn scene. Vanishing lines run from the foreground to one or more vanishing points on the horizon line.