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.

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.

A plane parallel to the sensible horizon of a place, and
passing through the earth’s center; – called also rational / celestial
horizon.

The unbroken line separating sky and water, as seen by an
eye at a given elevation, no land being visible.

The epoch or time during which a deposit was made.

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.