horizontal
n.n. flat and level, like the ground or the line where the sea meets the sky. It goes from side to side rather than up and down.
n. parallel to the plane of the horizon; at right angles to the vertical. Often used to describe the orientation of lines, surfaces, or physical posture.
Draw a horizontal line across the middle of the page.
The architect insisted on using horizontal wooden planks to make the small building appear wider than it actually was.
In many sedimentary rock formations, the layers remain perfectly horizontal for miles unless tectonic forces eventually tilt or fold them into new orientations.
Borrowed from Middle French horizontal, from horizon, horizonte, from Latin horizōn (stem horizont-).
Typically used as an attributive adjective before a noun, though it can also function as a predicative adjective after a linking verb.