If you're sitting at a table, you'll have to bring your hands above the horizontal plane of the tabletop.

FORBES: Magazine Article

The Olympus SZ-12 offers the new Reflection magic filter takes the image and duplicates it across a horizontal or vertical plane of symmetry creating powerful illusions.

ENGADGET: Olympus unveils a cavalcade of point and shoot cameras and a PEN lens at CES

This is a special type of Earth model which traces its idealised "horizontal" surface - the plane on which, at any point, the pull of gravity is perpendicular to it.

BBC: Europe's Goce satellite probes Earth's gravity