horizon - n.
1 a the line at which the earth and sky appear to meet. b (in full apparent or sensible or visible horizon) the line at which the earth and sky would appear to meet but for irregularities and obstructions; a circle where the earth's surface touches a cone whose vertex is at the observer's eye. c (in full celestial or rational or true horizon) 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 limit of mental perception, experience, interest, etc.
3 a geological stratum or set of strata, or layer of soil, with particular characteristics.
4 Archaeol. the level at which a particular set of remains is found.

on the horizon (of an event) just imminent or becoming apparent. [ME f. OF orizon(te) f. LL horizon -ontis f. Gk horizon (kuklos) limiting (circle)]