the line or circle that forms the apparent boundary between earth and sky.

Astronomy .

the small circle of the celestial sphere whose plane is tangent to the earth at the position of a given observer, or the plane of such a circle (sensible horizon).

Also called rational horizon. the great circle of the celestial sphere whose plane passes through the center of the earth and is parallel to the sensible horizon of a given position, or the plane of such a circle (celestial horizon).

the limit or range of perception, knowledge, or the like.

Usually, horizons. the scope of a person's interest, education, understanding, etc.

His horizons were narrow.

Geology . a thin, distinctive stratum useful for stratigraphic correlation.

any of the series of distinctive layers found in a vertical cross section of any well-developed soil.

noun

border , boundary , compass , field of vision , ken , limit , perspective , prospect , purview , range , reach , realm , scope , sphere , stretch , vista , edge , future , outlook , skyline