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

plural of horizon

(philosophy) Relating to a horizon.

(figuratively) Imminent.

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

(geology, soil science) The formation of soil horizons

(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.

plural of event horizon

(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.