adj. & n.
adj. 1 a parallel to the plane of the horizon, at right angles to the vertical (horizontal plane). b (of machinery etc.) having its parts working in a horizontal direction. 2 a combining firms engaged in the same stage of production (horizontal integration). b involving social groups of equal status etc. 3 of or at the horizon.
n. a horizontal line, plane, etc. horizontality n. horizontally adv. horizontalness n.
[ F horizontal or mod.L horizontalis (as HORIZON) ]