S: (n) straight line (a line traced by a point traveling in a constant direction; a line of zero curvature) "the shortest distance between two points is a straight line"
W: (adj) parallel [Indirect via oblique] (being everywhere equidistant and not intersecting) "parallel lines never converge"; "concentric circles are parallel"; "dancers in two parallel rows"
W: (adj) perpendicular [Indirect via oblique] (intersecting at or forming right angles) "the axes are perpendicular to each other"