The Collaborative International Dictionary of English v.0.48:

Invagination \In*vag`i*na"tion\, n. [L. pref. in- + vagina
   sheath.]
   [1913 Webster]
   1. (Biol.) The condition of an invaginated organ or part.
      [1913 Webster]

   2. The inward movement of the wall of a tissue or cell, to
      form a cavity; also, the cavity thus formed.
      [PJC]

   3. Specifically: (Biol., Embryology) The inward movement of
      one part of the wall of a blastula, to form a gastrula;
      the process of gastrulation, in which layers of the ovum
      are differentiated.
      [1913 Webster +PJC]

   Note: In embolic invagination, one half of the blastosphere
         is pushed in towards the other half, producing an
         embryonic form known as a gastrula. -- In epibolic
         invagination, a phenomenon in the development of some
         invertebrate ova, the epiblast appears to grow over or
         around the hypoblast.
         [1913 Webster]

WordNet (r) 3.0 (2006):

invagination
    n 1: the condition of being folded inward or sheathed [syn:
         invagination, introversion]
    2: 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]