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]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "taciturn":
   Spartan, abbreviated, abridged, aposiopestic, brief, brooding,
   brusque, clipped, close, close-lipped, close-tongued, closemouthed,
   compact, compendious, compressed, concise, condensed, contracted,
   crisp, curt, cut, docked, dour, dumb, economical of words,
   elliptic, epigrammatic, gnomic, indisposed to talk, laconic, mum,
   mute, pithy, pointed, pruned, quiet, reserved, reticent,
   sententious, short, short and sweet, shortened, silent, snug,
   sparing of words, speechless, succinct, summary, synopsized, terse,
   tight, tight-lipped, to the point, tongue-tied, truncated,
   uncommunicative, unexpressive, unloquacious, untalkative,
   word-bound, wordless