From Greek 'horizōn' (bounding), from 'horos' (boundary) — the Greeks saw it as the 'bounding circle,' the limit of visible space.
The line at which the earth's surface and the sky appear to meet; figuratively, the limit of one's experience, knowledge, or ambition.
From Old French 'orizon,' from Latin 'horizōn,' from Greek 'horizōn (kyklos)' meaning 'bounding (circle),' the present participle of 'horizein' (to bound, to limit, to define), from 'horos' (boundary, limit, landmark). The Greeks conceived the horizon as the bounding circle of one's vision — the line that defines the limit of what can be seen. The word entered English through astronomical and navigational texts during the late medieval period. Key roots: horizein (Greek: "to bound, to limit, to define"), horos (Greek: "boundary, limit, landmark").
The word 'horoscope' is an etymological cousin of 'horizon' — both come from Greek 'horos' (boundary). A horoscope literally means 'boundary-watcher' or 'hour-observer' (from 'horos' + 'skopein,' to watch), because it observes the position of celestial bodies at the boundary moment of one's birth.