The Collaborative International Dictionary of English v.0.48:

Regress \Re"gress\ (r?"gr?s), n. [L. regressus, fr. regredi,
   regressus. See Regrede.]
   1. The act of passing back; passage back; return;
      retrogression. "The progress or regress of man". --F.
      Harrison.
      [1913 Webster]

   2. The power or liberty of passing back. --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Regress \Re*gress"\ (r?*gr?s"), v. i. [imp. & p. p. Regressed
   (-gr?st"); p. pr. & vb. n. Regressing.]
   To go back; to return to a former place or state. --Sir T.
   Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

regress
    n 1: the reasoning involved when you assume the conclusion is
         true and reason backward to the evidence [syn: regress,
         reasoning backward]
    2: returning to a former state [syn: regression, regress,
       reversion, retrogression, retroversion]
    v 1: go back to a statistical means
    2: go back to a previous state; "We reverted to the old rules"
       [syn: revert, return, retrovert, regress, turn
       back]
    3: 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]
    4: 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:

129 Moby Thesaurus words for "regress":
   about-face, advance, ascend, back, back up, backing, backset,
   backslide, backsliding, backward motion, backward step, budge,
   change, change place, circle, climb, cock, come after, come last,
   degenerate, descend, deteriorate, disenchantment, disimprove, ebb,
   fall, fall again into, fall astern, fall back, fall behind,
   fall from grace, flip-flop, flow, follow, get behind, get over,
   get worse, go, go around, go back, go backwards, go behind,
   go round, go sideways, grow worse, gyrate, have a relapse,
   jerk back, lag behind, lapse, lapse back, let down, lose ground,
   mount, move, move over, plunge, progress, pull back, reaction,
   recede, recession, recidivate, recidivation, recidivism,
   reclamation, reconversion, recur to, reentry, refluence, reflux,
   regression, rehabilitation, reinstatement, relapse, restitution,
   restoration, retroaction, retrocede, retrocession, retroflex,
   retroflexion, retrogradation, retrograde, retrogress,
   retrogression, retroversion, retrovert, retrusion, return,
   return to, returning, reversal, reverse, reversion, revert,
   revert to, reverting, revulsion, rise, rollback, rotate, run,
   setback, shift, sicken, sink, sink back, slacken, slide back,
   slip back, slipping back, soar, spin, sternway, stir, stream,
   subside, throw back, throwback, trail, trail behind, travel, turn,
   turnabout, wane, whirl, worsen, yield again to



Bouvier's Law Dictionary, Revised 6th Ed (1856):

REGRESS. Returning; going back opposed to ingress. (q.v.)