Antonyms for Perpendicular (opposite meaning)
WordNet sense 1 (slanting or inclined in direction or course or position--neither parallel nor perpendicular nor right-angled):
obliqueWordNet sense 2 (being everywhere equidistant and not intersecting):
parallelWordNet sense 3 (at an angle to the horizontal or vertical position):
inclinedWordNet sense 4 (parallel to or in the plane of the horizon or a base line):
horizontalFrom the ODE community, based on WordNetadd/edit