An Etymological Dictionary of Astronomy and Astrophysics

A line or plane at right angles to another line or plane. Two curves are said to be perpendicular if their tangent lines are mutually perpendicular. → normal; → vertical

Etymology (EN): From M.E. perpendiculer(e), from
O.Fr. perpendiculiere, from L. perpendicularis “vertical, as a plumb line,” from perpendiculum “plumb line,” from perpendere “balance carefully,” from per- “thoroughly” + pendere “to weigh, to hang.”

Etymology (PE): Pâlâr “pillar, column, main beam.”