Horizontal

[hawr-uh-zon-tl, hor-] /ˌhɔr əˈzɒn tl, ˌhɒr-/

adjective
1.
at right angles to the vertical; parallel to level ground.
2.
flat or level:
a horizontal position.
3.
being in a prone or supine position; recumbent:
His bad back has kept him horizontal for a week.
4.
near, on, or parallel to the horizon.
5.
of or relating to the horizon.
6.
measured or contained in a plane parallel to the horizon:
a horizontal distance.
7.
(of material on a printed page, pieces on a game board, etc.) extending across, from the left to the right of the viewer.
8.
of or relating to a position or individual of similar status:
He received a horizontal promotion to a different department, retaining his old salary and title.
9.
Economics. of or relating to companies, affiliates, divisions, etc., that perform the same or similar functions or produce the same or similar products:
Through horizontal mergers the company monopolized its field.
noun
10.
anything horizontal, as a plane, direction, or object.
/ˌhɒrɪˈzɒntəl/
adjective
1.
parallel to the plane of the horizon; level; flat Compare vertical (sense 1)
2.
of or relating to the horizon
3.
measured or contained in a plane parallel to that of the horizon
4.
applied uniformly or equally to all members of a group
5.
(economics) relating to identical stages of commercial activity: horizontal integration
noun
6.
a horizontal plane, position, line, etc
adj.

1550s, “relating to or near the horizon,” from French horizontal, from Latin horizontem (see horizon). Meaning “flat” (i.e., “parallel to the horizon”) is from 1630s. Related: horizontally.