The Collaborative International Dictionary of English v.0.48:

Involution \In`vo*lu"tion\, n. [L. involutio: cf. F. involution.
   See Involve.]
   [1913 Webster]
   1. The act of involving or infolding.
      [1913 Webster]

   2. The state of being entangled or involved; complication;
      entanglement.
      [1913 Webster]

            All things are mixed, and causes blended, by mutual
            involutions.                          --Glanvill.
      [1913 Webster]

   3. That in which anything is involved, folded, or wrapped;
      envelope. --Sir T. Browne.
      [1913 Webster]

   4. (Gram.) The insertion of one or more clauses between the
      subject and the verb, in a way that involves or
      complicates the construction.
      [1913 Webster]

   5. (Math.) The act or process of raising a quantity to any
      power assigned; the multiplication of a quantity into
      itself a given number of times; -- the reverse of
      evolution.
      [1913 Webster]

   6. (Geom.) The relation which exists between three or more
      sets of points, a.a', b.b', c.c', so related to a point O
      on the line, that the product Oa.Oa' = Ob.Ob' = Oc.Oc' is
      constant. Sets of lines or surfaces possessing
      corresponding properties may be in involution.
      [1913 Webster]

   7. (Med.) The return of an enlarged part or organ to its
      normal size, as of the uterus after pregnancy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

involution
    n 1: reduction in size of an organ or part (as in the return of
         the uterus to normal size after childbirth)
    2: a long and intricate and complicated grammatical construction
    3: marked by elaborately complex detail [syn: elaborateness,
       elaboration, intricacy, involution]
    4: the act of sharing in the activities of a group; "the teacher
       tried to increase his students' engagement in class
       activities" [syn: engagement, participation,
       involvement, involution] [ant: non-engagement, non-
       involvement, nonparticipation]
    5: the process of raising a quantity to some assigned power
       [syn: exponentiation, involution]
    6: the action of enfolding something [syn: involution,
       enfolding]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "involution":
   absorption, addition, ambages, anfractuosity, approximation,
   circuitousness, circumambages, circumbendibus, circumlocution,
   circumvolution, comedown, complexity, complexness, complication,
   convolution, crabbedness, crinkle, crinkling, debasement,
   decadence, decadency, declension, declination, decline,
   deformation, degeneracy, degenerateness, degeneration, degradation,
   demotion, depravation, depravedness, depreciation, derogation,
   descent, deterioration, devolution, differentiation, division,
   downtrend, downturn, downward mobility, downward trend, drop,
   dying, ebb, effeteness, embarrassment, engagement, enmeshment,
   entanglement, equation, evolution, extrapolation, fading, failing,
   failure, failure of nerve, fall, falling-off, flexuosity,
   flexuousness, implication, inclusion, integration, interpolation,
   intorsion, intricacy, intricateness, inversion, involvement, lapse,
   loss of tone, meander, meandering, multiplication, notation,
   perplexity, practice, proportion, ramification, reduction,
   regression, relation, retrocession, retrogradation, retrogression,
   rivulation, sinuation, sinuosity, sinuousness, slinkiness,
   slippage, slump, snakiness, subtlety, subtraction, tanglement,
   technicality, torsion, tortility, tortuosity, tortuousness,
   transformation, turning, twisting, undulation, wane, wave, waving,
   winding