introversion

(noun) the condition of being folded inward or sheathed. Synonyms: invagination.

(noun) the folding in of an outer layer so as to form a pocket in the surface. Synonyms: invagination, intussusception, infolding. Example: "The invagination of the blastula."

(noun) (psychology) an introverted disposition; concern with one's own thoughts and feelings.