horizon

Concise Oxford English Dictionary © 2008 Oxford University Press:
horizon/həˈrʌɪzn/

▶noun

• 1 the line at which the earth's surface and the sky appear to meet.

  ■ (also apparent horizon) this line represented as a circle, 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.

• 2 the limit of a person's knowledge, experience, or interest.

• 3 Geology & Archaeology a layer or level of soil or rock with particular characteristics or representing a particular period.

– phrases
on the horizon imminent.

– origin ME: via OFr. from late L. horizon, from Gk horizōn (kuklos) ‘limiting (circle)’.