noun

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