Lines that meet at right angles are described as perpendicular to each other. Another word that describes lines that meet at right angles is orthogonal.
In two-dimensional plane geometry, whether or not two lines are perpendicular is determined by taking the product of the slopes. If the two lines are perpendicular, the product of the slopes is -1. For instance, a line with slope 5 is perpendicular to any line with slope -1/5. Any vertical line is defined as perpendicular to any horizontal line. This rule is necessary because the slope of a vertical line is undefined, and the slope of a horizontal line is zero.