The visible horizontal line (in all directions) where the sky appears to meet the earth in the distance.

plural of horizon

(figuratively) Imminent.

(soil science) The topmost layer of the soil horizons; topsoil

(philosophy) Relating to a horizon.

(geology, soil science) The formation of soil horizons

plural of marker horizon

(geochronology) A stratigraphic unit of distinctive lithology with a wide geographical extent used in stratigraphic correlation.

(astrophysics) The gravitational sphere of a black hole within which the escape velocity is greater than the speed of light. / (by extension) A point of no return.

(aviation) An instrument that displays the pitch and roll of an aircraft in flight by comparing its attitude to a gyroscopically-maintained indication of the flat horizon line.