The Collaborative International Dictionary of English v.0.48:

Retrogress \Re"tro*gress\, n. [Cf. L. retrogressus.]
   Retrogression. [R.] --H. Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

retrogress
    v 1: get worse or fall back to a previous condition [syn:
         regress, retrograde, retrogress] [ant: advance,
         come along, come on, get along, get on, progress,
         shape up]
    2: go back to bad behavior; "Those who recidivate are often
       minor criminals" [syn: relapse, lapse, recidivate,
       regress, retrogress, fall back]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "retrogress":
   advance, ascend, back, back up, backslide, budge, change,
   change place, circle, climb, cock, come after, come last,
   degenerate, descend, deteriorate, disimprove, ebb, fall astern,
   fall back, fall behind, flow, follow, get behind, get over,
   get worse, go, go around, go back, go backwards, go behind,
   go round, go sideways, grow worse, gyrate, jerk back, lag behind,
   lapse, lapse back, let down, lose ground, mount, move, move over,
   plunge, progress, pull back, recede, recidivate, regress, relapse,
   retrocede, retroflex, retrograde, retrovert, return, reverse,
   revert, rise, rotate, run, shift, sicken, sink, slacken, slip back,
   soar, spin, stir, stream, subside, throw back, trail, trail behind,
   travel, wane, whirl, worsen