The Collaborative International Dictionary of English v.0.48:

Retrogression \Re`tro*gres"sion\, n. [Cf. F. r['e]trogression.
   See Retrograde, and cf. Digression.]
   1. The act of retrograding, or going backward;
      retrogradation.
      [1913 Webster]

   2. (Biol.) Backward development; a passing from a higher to a
      lower state of organization or structure, as when an
      animal, approaching maturity, becomes less highly
      organized than would be expected from its earlier stages
      or known relationship. Called also retrograde
      development, and regressive metamorphism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

retrogression
    n 1: passing from a more complex to a simpler biological form
         [syn: degeneration, retrogression]
    2: returning to a former state [syn: regression, regress,
       reversion, retrogression, retroversion]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "retrogression":
   Brownian movement, about-face, advance, angular motion, ascending,
   ascent, axial motion, backflowing, backing, backset, backsliding,
   backward motion, backward step, career, climbing, comedown, course,
   current, debasement, decadence, decadency, declension, declination,
   decline, deformation, degeneracy, degenerateness, degeneration,
   degradation, demotion, depravation, depravedness, depreciation,
   derogation, descending, descent, deterioration, devolution,
   disenchantment, downtrend, downturn, downward mobility,
   downward motion, downward trend, drift, driftage, drop, dying, ebb,
   ebbing, effeteness, fading, failing, failure, failure of nerve,
   fall, falling-off, flight, flip-flop, flow, flux, forward motion,
   involution, lapse, loss of tone, mounting, oblique motion, ongoing,
   onrush, passage, plunging, progress, radial motion, random motion,
   reaction, recession, recidivation, recidivism, reclamation,
   reconversion, reentry, reflowing, refluence, reflux, regress,
   regression, rehabilitation, reinstatement, relapse, restitution,
   restoration, retroaction, retrocession, retroflexion,
   retrogradation, retroversion, retrusion, return, returning,
   reversal, reverse, reversion, reverting, revulsion, rising,
   rollback, run, rush, set, setback, sideward motion, sinking,
   slippage, slipping back, slump, soaring, sternway, stream,
   subsiding, throwback, traject, trajet, trend, turn, turnabout,
   upward motion, wane