Merriam-Webster Collegiate Dictionary - horizon

 noun  Etymology: Middle English orizon, from Late Latin ~t-, ~, from Greek ~t-, horizon, from present participle of horizein to bound, define, from horos boundary; perhaps akin to Latin urvum curved part of a plow  Date: 14th century  1.  a. the apparent junction of earth and sky  b. the great circle on the celestial sphere formed by the intersection of the celestial sphere with a plane tangent to the earth's surface at an observer's position — see azimuth illustration  c. range of perception or experience  d. something that might be attained new ~s  2.  a. the geological deposit of a particular time usually identified by distinctive fossils  b. any of the reasonably distinct layers of soil or its underlying material in a vertical section of land  c. a cultural area or level of development indicated by separated groups of artifacts  • ~al adjective