The Collaborative International Dictionary of English v.0.48:

Intussusception \In`tus*sus*cep"tion\, n. [L. intus within +
   susception. Cf. Introsusception.]
   [1913 Webster]
   1. The reception of one part within another.
      [1913 Webster]

   2. (Med.) The abnormal reception or slipping of a part of a
      tube, by inversion and descent, within a contiguous part
      of it; specifically, the reception or slipping of the
      upper part of the small intestine into the lower;
      introsusception; invagination. --Dunglison.
      [1913 Webster]

   3. (Bot.) The interposition of new particles of formative
      material among those already existing, as in a cell wall,
      or in a starch grain.
      [1913 Webster]

   4. (Physiol.) The act of taking foreign matter, as food, into
      a living body; the process of nutrition, by which dead
      matter is absorbed by the living organism, and ultimately
      converted into the organized substance of its various
      tissues and organs.
      [1913 Webster]

            Dead bodies increase by apposition; living bodies by
            intussusception.                      --McKendrick.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intussusception
    n 1: the folding in of an outer layer so as to form a pocket in
         the surface; "the invagination of the blastula" [syn:
         invagination, introversion, intussusception,
         infolding]
    2: (biology) growth in the surface area of a cell by the deposit
       of new particles between existing particles in the cell wall