horizon

Etymology

From Old French orizon, via Latin horizōn, from Ancient Greek ὁρίζων (horízōn), from ὅρος (hóros, “boundary”).

noun

1. The visible horizontal line (in all directions) where the sky appears to meet the earth in the distance.
   A tall building was visible on the horizon.
2. (figurative) The range or limit of one's knowledge, experience or interest; a boundary or threshold.
   Some students take a gap year after finishing high school to broaden their horizons.
   With clinical researchers hard at work, a new treatment is on the horizon.
   The Indians of the Americas totaled no less than 70 million when the foreign conquerors appeared on the horizon; a century and a half later they had been reduced to 3.5 million. 1997, Eduardo Galeano, Open Veins of Latin America: Five Centuries of the Pillage of a Continent, Monthly Review Press, page 38
3. The range or limit of any dimension in which one exists.
   Only mortality, this irreducible and primordial horizon, that very horizon which, in Being and Time, Heidegger so compellingly revealed as the unsurpassable and defining possibility, remains. 2003, Miguel de Beistegui, Thinking with Heidegger: Displacements, page 157
4. (geology) A specific layer of soil, or stratum
5. (archaeology, chiefly US) A cultural sub-period or level within a more encompassing time period.
6. Any level line or surface.
7. (chess) The point at which a computer chess algorithm stops searching for further moves.