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

(geology, soil science) The formation of soil horizons

(philosophy) Relating to a horizon.

(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.

(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.

(astronomy) The distance from an observer in a universe beyond which it is impossible for the observer to see, as the light (or other luminal particles) cannot have travelled that distance over the current age of that universe.

plural of event horizon