Perpendicular Lines
Perpendicular lines are two lines that intersect to form a right angle (exactly 90 degrees). The point where they cross is called the point of intersection.
Example
In coordinate geometry, two non-vertical lines are perpendicular if the product of their gradients is $-1$, i.e., $m_1 \times m_2 = -1$. For example, $y = \frac{1}{2}x$ and $y = -2x$ are perpendicular lines.