horizon

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 whole sweep of the horizon.”

2. (figuratively)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.”

   “It is because theirs has become the only explicit general philosophy of social policy held by a large group, the only system or theory which raises new problems and opens new horizons, that they have succeeded in inspiring the imagination of the intellectual.”

   “They learn that reading, besides being something one does at school, is also something one can do on one's own, for fun, to satisfy curiosity, or even to "expand one's horizons.””

   “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.”

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.”

4. A specific layer of soil, or stratum
5. (US)A cultural sub-period or level within a more encompassing time period.
6. Any level line or surface.
7. The point at which a computer chess algorithm stops searching for further moves.