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

(figurative) The range or limit of one's knowledge, experience or interest; a boundary or threshold.

The range or limit of any dimension in which one exists.

(geology) A specific layer of soil, or stratum

(archaeology, chiefly US) A cultural sub-period or level within a more encompassing time period.

Any level line or surface.

(chess) The point at which a computer chess algorithm stops searching for further moves.