The Collaborative International Dictionary of English v.0.48:

Discreet \Dis*creet"\, a. [Compar. Discreeter; superl.
   Discreetest.] [F. discret, L. discretus separated (whence
   the meaning reserved, prudent), p. p. of discernere. See
   Discern, and cf. Discrete.]
   1. Possessed of discernment, especially in avoiding error or
      evil, and in the adaptation of means to ends; prudent;
      sagacious; judicious; not rash or heedless; cautious.
      [1913 Webster]

            It is the discreet man, not the witty, nor the
            learned, nor the brave, who guides the conversation,
            and gives measures to society.        --Addison.
      [1913 Webster]

            Satire 's my weapon, but I 'm too discreet
            To run amuck, and tilt at all I meet. --Pope.
      [1913 Webster]

            The sea is silent, the sea is discreet.
                                                  --Longfellow.
      [1913 Webster]

   2. Differing; distinct. [Obs.] --Spenser. -- Dis*creet"ly,
      adv. -- Dis*creet"ness, n. Discrepance

WordNet (r) 3.0 (2006):

discreet
    adj 1: marked by prudence or modesty and wise self-restraint;
           "his trusted discreet aide"; "a discreet, finely wrought
           gold necklace" [ant: indiscreet]
    2: unobtrusively perceptive and sympathetic; "a discerning
       editor"; "a discreet silence" [syn: discerning, discreet]
    3: heedful of potential consequences; "circumspect actions";
       "physicians are now more circumspect about recommending its
       use"; "a discreet investor" [syn: circumspect, discreet]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "discreet":
   Olympian, aloof, backward, bashful, blank, calculating, canny,
   careful, cautious, chary, chilled, chilly, circumspect, close,
   closemouthed, cold, considerate, constrained, controlled, cool,
   dark, deliberate, detached, diplomatic, discriminating,
   discriminative, distant, dry, enlightened, evasive, expressionless,
   forbidding, frigid, frosty, gingerly, guarded, heedful, hesitant,
   icy, impassive, impersonal, inaccessible, introverted, judgmatic,
   judicial, judicious, leaving out nothing, mindful, moderate,
   modest, noncommittal, offish, on guard, overlooking no possibility,
   pawky, politic, provident, prudent, prudential, reasonable,
   reflecting, reflective, regardful, remote, removed, repressed,
   reserved, restrained, reticent, retiring, safe, secret, secretive,
   shifty, shrinking, simple, slow to act, standoff, standoffish,
   subdued, suppressed, tactful, temperate, tentative, thorough,
   thoughtful, unadorned, unadventurous, unaffable, unapproachable,
   uncommunicative, uncongenial, undaring, undemonstrative,
   unelaborate, unenterprising, unexcessive, unexpansive, unextreme,
   ungenial, unostentatious, unprecipitate, unpretentious, wary,
   watchful, well-advised, well-judged, withdrawn