The Collaborative International Dictionary of English v.0.48:

Rectilineal \Rec`ti*lin"e*al\ (-l?n"?*al), Rectilinear
\Rec`ti*lin"e*ar\ (-l?n"?*?r), a. [Recti- + lineal, linear.]
   Straight; consisting of a straight line or lines; bounded by
   straight lines; as, a rectineal angle; a rectilinear figure
   or course. -- Rec`ti*lin"e*al*ly, adv. --
   Rec`ti*lin"e*ar*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rectilinear
    adj 1: characterized by a straight line or lines; "rectilinear
           patterns in wallpaper"; "the rectilinear propagation of
           light" [syn: rectilinear, rectilineal]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "rectilinear":
   arrowlike, dead straight, direct, even, flat, horizontal,
   in a line, level, lineal, linear, rectilineal, right,
   ruler-straight, smooth, straight, straight-cut, straight-front,
   straight-side, streamlined, true, unbending, unbent, unbowed,
   unbroken, uncurved, undeflected, undeviating, undistorted,
   uninterrupted, unswerving, unturned, upright, vertical