(1):

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

(2):

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

(3):

(a.) 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.

(4):

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