Perpendicular  —adj. 1 a (usu. Foll. By to) at right angles (to a given line, plane, or surface). B at right angles to the plane of the horizon. 2 upright, vertical. 3 (of a slope etc.) Very steep. 4 (perpendicular) archit. Of the third stage of english gothic (15th–16th c.) With vertical tracery in large windows. —n. 1 perpendicular line. 2 (prec. By the) perpendicular line or direction (is out of the perpendicular).  perpendicularity n. [latin perpendiculum plumb-line]