horizontal
//ˌhɒrɪˈzɒntəl//
Adjective
1
parallel to the ground or to the horizon; flat and level
Draw a horizontal line across the page.
2
relating to or involving people or things at the same level in a hierarchy
The company encourages horizontal communication between departments.
Antonym
Noun
1
a horizontal line, plane, or position
The carpenter checked that the shelf was perfectly on the horizontal.