Orthogonal lines are lines that intersect at a right angle, which is 90 degrees. In geometry, this concept is important because it helps define shapes and angles. When two lines are orthogonal, they create a clear and distinct relationship, often used in various fields such as architecture and engineering. In a coordinate system, such as the Cartesian plane, orthogonal lines can be represented by slopes that are negative reciprocals of each other. For example, if one line has a slope of 2, an orthogonal line would have a slope of -1/2. This property is useful in mathematics and design, ensuring structures are stable and visually appealing.