A horizontal line is a line that runs from left to right across a page or is parallel to the horizon. It is flat and level with the ground, and it is the opposite of a vertical line, which runs up and down the page. In geometry, a horizontal line is one that runs from left to right across the page, and it is parallel to the horizon. The word "horizontal" is derived from the Latin "horizon," which means "separating" or "marking a boundary".
In general, something that is horizontal can be drawn from left to right, such as the x-axis in the Cartesian coordinate system. The concept of horizontality only makes sense in the context of a clearly measurable gravity field, such as in the neighborhood of a planet, star, etc. When the gravity field becomes very weak, the notion of being horizontal loses its meaning.
In summary, a horizontal line is parallel to the horizon and runs from left to right across a page or is level with the ground. It is the opposite of a vertical line, which runs up and down the page.
