Lying flat or parallel to the ground or horizon; going from left to right rather than up and down.
From Greek 'horizont' meaning 'bounding circle,' from 'horizein' meaning 'to bound.' Latin adopted this as 'horizontalis.' The word literally referred to the line where earth meets sky.
The horizon line was so important to human navigation and art that ancient Greeks named it—and it's one of the reasons humans naturally divide space into horizontal and vertical, which is why paintings look wrong when the horizon is tilted; our brains are literally wired by gravity to see horizontals.
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