1 [navigation] The apparent or visible junction of land and sky.
2 [astronomy] The horizontal plane tangent to the earth’s surface and perpendicular to the line through an observer’s position and the zenith of that position. The apparent or visible horizon approximates the true horizon only when the point of vision is very close to sea level.
3 [astronomy] The great circle in which an observer’s horizon meets the celestial sphere.
4 [map projections] The edge of a map projection.