horizon

noun

• 1the line at which the earth’s surface and the sky appear to meet:the sun rose above the horizon
• (also apparent horizon) the circular boundary of the part of the earth’s surface visible from a particular point, ignoring irregularities and obstructions.
• (also true horizon) Astronomy a great circle of the celestial sphere, the plane of which passes through the centre of the earth and is parallel to that of the apparent horizon of a place.

• 2 (often horizons) the limit of a person’s knowledge, experience, or interest:she wanted to leave home and broaden her horizons

• 3 Geology a layer of soil or rock, or a set of strata, with particular characteristics.
• Archaeology a level of an excavated site representing a particular period: the upper horizon of the site showed an arrangement of two rows of features