Origin and history of horizontal

late 14c., orisoun, from Old French orizon (14c., Modern French horizon), earlier orizonte (13c.), from Latin horizontem (nominative horizon), from Greek horizon (kyklos) "bounding (circle)," from horizein "bound, limit, divide, separate," from horos "boundary, landmark, marking stones." Beekes connects this with Latin -urvare "to mark out a boundary with a furrow," urvum "curved part of a plough," perhaps from a PIE root meaning "to draw." 

The h- was restored in English 17c. in imitation of Latin (see H). Old English used eaggemearc ("eye-mark") for "limit of view, horizon." The apparent horizon is distinguished from the celestial or astronomical horizon.