A horizontal line is a straight line that runs from left to right, parallel to the horizon. It has a constant y-coordinate, meaning it does not rise or fall as it extends. In geometry, horizontal lines are often used to represent stable or static conditions. In the Cartesian coordinate system, a horizontal line can be expressed with the equation y = c, where c is a constant. This line intersects the y-axis at the point (0, c) and is essential in various fields, including mathematics, art, and design, to create balance and structure.