1. (a.) Exactly upright or vertical; at right angles to the horizon or another object\; extending in a right line from any point toward the center of the earth.

2. (a.) At right angles to a given line or surface; as, the line ad is perpendicular to the line bc.

3. (n.) A line at right angles to the plane of the horizon; a vertical line or direction.

4. (n.) A line or plane falling at right angles on another line or surface, or making equal angles with it on each side.

air line axis azimuth circle beeline chord diagonal diameter direct line directrix edge erect great-circle course normal orthodiagonal orthogonal orthometric plumb plunging precipitous radius radius vector rectangular right angle right line right-angle right-angled right-angular secant segment sheer shortcut side stand-up steep straight straight course straight line straight stretch straight-up straight-up-and-down straightaway streamline tangent transversal up-and-down upright vector vertical vertical circle