The Collaborative International Dictionary of English v.0.48:

Taciturn \Tac"i*turn\, a. [L. taciturnus: cf. F. taciturne. See
   Tacit.]
   Habitually silent; not given to converse; not apt to talk or
   speak. -- Tac"i*turn*ly, adv.
   [1913 Webster]

   Syn: Silent; reserved.

   Usage: Taciturn, Silent. Silent has reference to the act;
          taciturn, to the habit. A man may be silent from
          circumstances; he is taciturn from disposition. The
          loquacious man is at times silent; one who is taciturn
          may now and then make an effort at conversation.
          [1913 Webster]

WordNet (r) 3.0 (2006):

taciturn
    adj 1: habitually reserved and uncommunicative [ant: voluble]