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.
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   3. That in which anything is involved, folded, or wrapped;
      envelope. --Sir T. Browne.
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   4. (Gram.) The insertion of one or more clauses between the
      subject and the verb, in a way that involves or
      complicates the construction.
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   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.
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   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.
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   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]