horizontal

Horizontal describes anything oriented, extended, or occurring parallel to the horizon. The term is used across science, engineering, design, and everyday language to denote a flat or level direction that runs left to right. It is typically understood as the opposite of vertical, which points upward and downward.

In geometry, a horizontal line has zero slope; its equation in a standard Cartesian plane is y = c, where c is a constant. A horizontal plane is a two-dimensional surface parallel to the horizon and perpendicular to the vertical axis.

In geography and surveying, horizontal often refers to the horizontal plane, used as a reference for measurements of distance and position. Horizontal coordinates, such as latitude and longitude, locate a point with reference to this plane, though practical measurements may use map projections that distort true three-dimensional space.

In science and technology, the term appears in phrases such as horizontal scaling (adding more machines to increase capacity), horizontal gene transfer in biology (exchange of genetic material between organisms without reproduction), and horizontal integration in business (merging with others at the same level).

In media, design, and photography, horizontal orientation describes images or layouts wider than they are tall, commonly associated with landscape formats. Videos and screens may be described as horizontal, in contrast to portrait (vertical) orientation.