Horizontal means flat and level, like the line where sky meets sea. A horizontal line, horizontal surface, lay it horizontal.
It is defined against vertical, and the pair covers most of how anything is described as lying. In organisations, a horizontal structure has few layers of management, against a vertical or hierarchical one. Horizontal integration means buying competitors at the same stage of a business rather than up or down the chain. Note horizontally; and that the word comes straight from horizon.
1adjectivehorizontalFlat and level, parallel to the horizon.
“Hang the picture so that the frame is perfectly horizontal.”
2adjectiveratherAcross one level rather than up and down a hierarchy.
“The horizontal structure has roughly two layers rather than nine.”
horizontal is an orientation.
Latin horizon came from Greek horizon kuklos, the bounding circle, from horizein, to bound or divide. A horizontal line is one parallel to the horizon.
Latin
horizōn
Middle French
horizontal
English, today
horizontal
Middle French horizontal, from horizon, horizonte, from Latin horizōn (stem horizont-).
Etymology from Wiktionary, CC BY-SA