Perpendicular lines are two lines that intersect at a right angle, which measures 90 degrees. This relationship is important in geometry and can be found in various shapes, such as squares and rectangles. When two lines are perpendicular, they create four right angles at the point of intersection. In a coordinate 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 for solving problems in geometry and trigonometry.