Horizon

noun

1. The visible horizontal line or point (in all directions) that appears to connect the Earth to the sky.

   "A tall building was visible on the horizon."

2. 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."

3. The range or limit of any dimension in which one exists.
4. A specific layer of soil or strata
5. A cultural sub-period or level within a more encompassing time period.