The term "horizon" may refer to any of the following:
The apparent line where the sky meets the land or sea. At sea, its distance varies in proportion to the square root of the height of the observer's eyes above sea level: if this is, say, 2 meters the horizon will be about 5.57 kilometers distant.
Astronomical horizon (or sensible horizon): the great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith; in other words, the great circle whose poles are the nadir and zenith.
Cosmological horizon: the maximum distance from us that light has traveled since the beginning of the universe; objects farther away are invisible to us because there has not been enough time for light to travel from them to Earth.