Definition of: invagination

(in·vaj′ə·nā′shən) noun
1. The act of invaginating, or the state of being invaginated; intussusception.
2. Biol. a The differentiation of the germinal layers by a pushing in of the wall of the blastula to form the gastrula, or by the growth of the epiblast cells as a thin layer over the hypoblast. b That which is invaginated; specifically, a pouch formed by an infolding of a membrane: the buccal invagination.