Perpendicular

1. Exactly upright or vertical; pointing to the zenith; at right angles to the plane of the horizon; extending in a right line from any point toward the center of the earth.
2. A line or plane falling at right angles on another line or surface, or making equal angles with it on each side.
3. At right angles to a given line or surface; as, the line ad is perpendicular to the line bc.
4. A line at right angles to the plane of the horizon; a vertical line or direction.