A perpendicular line is a line that intersects another line at a right angle, which is 90 degrees. This relationship is important in geometry, as it helps define various shapes and structures. When two lines are perpendicular, they create four right angles at the point of intersection. In a coordinate system, such as the Cartesian plane, perpendicular lines have slopes that are negative reciprocals of each other. For example, if one line has a slope of 2, a line perpendicular to it will have a slope of -1/2. Understanding perpendicular lines is essential in fields like architecture and engineering.