Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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}.

Source : WordNet®

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