Answers.com defines introvert as:
While Taciturn is:Verb.
1. To turn or direct inward.
2. Psychology. To concentrate (one's interests) upon oneself.
3. Medicine. To turn (a tubular organ or part) inward upon itself.
Noun.
1. Psychology. An introverted person.
2. Medicine. An anatomical structure that is capable of being introverted.
INTRO– + Latin vertere, to turn.
Adj.
Habitually untalkative. See synonyms at silent.
[French taciturne, from Old French, from Latin taciturnus, from tacitus, silent. See tacit.]