introvert

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To turn or direct inward.
• transitive verb Psychology To concentrate (one's interests) upon oneself.
• transitive verb Medicine To turn (a tubular organ or part) inward upon itself.
• noun Psychology An introverted person.
• noun Medicine An anatomical structure that is capable of being introverted.

from The Century Dictionary.

• noun That which is introverted; in zoology, some part or organ which is turned in upon itself, or intus-suscepted.
• To turn within; direct inward or interiorly.
• In zoology, to turn in, or invert; insheathe a part of within another part.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To turn or bend inward.
• transitive verb To look within; to introspect.
• noun A person who is introverted; one concerned predominantly with himself or his own feelings. Contrasted with extrovert.
• noun A person who is shy.
• noun (Zoöl.) A part that can be introverted{1}.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective of a person Who focuses primarily on their own mind, feelings, or affairs.
• noun One who focuses primarily on their own mind, feelings, or affairs.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (psychology) a person who tends to shrink from social contacts and to become preoccupied with their own thoughts
• verb fold inwards
• verb turn inside