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]