The term "perpendicular" refers to the relationship between two lines, segments, or planes that intersect at a right angle, which is 90 degrees. When one line is perpendicular to another, it means they meet in such a way that they form a square corner. This concept is fundamental in geometry and is often used in various fields, including architecture and engineering. In a coordinate system, such as the Cartesian plane, two lines are perpendicular if the product of their slopes is -1. For example, if one line has a slope of 2, a line perpendicular to it would have a slope of -1/2. Understanding perpendicularity is essential for constructing right angles and ensuring structural integrity in design.