In geometry, two lines or planes (or a line and a plane), are considered perpendicular (or orthogonal) to each other if they form congruent adjacent angles (an L-shape). The term may be used as a noun or adjective. Thus, referring to Figure 1, the line AB's the perpendicular to CD through the point B. Note that by definition, a line's infinitely long, and strictly speaking AB and CD in this example represent line segments of two infinitely long lines. Hence the line segment AB doesn't have to i… (More on Perpendicular)