The Collaborative International Dictionary of English v.0.48:

Betray \Be*tray"\ (b[-e]*tr[=a]"), v. t. [imp. & p. p.
   Betrayed (-tr[=a]d"); p. pr. & vb. n. Betraying.] [OE.
   betraien, bitraien; pref. be- + OF. tra["i]r to betray, F.
   trahir, fr. L. tradere. See Traitor.]
   1. To deliver into the hands of an enemy by treachery or
      fraud, in violation of trust; to give up treacherously or
      faithlessly; as, an officer betrayed the city.
      [1913 Webster]

            Jesus said unto them, The Son of man shall be
            betrayed into the hands of men.       --Matt. xvii.
                                                  22.
      [1913 Webster]

   2. To prove faithless or treacherous to, as to a trust or one
      who trusts; to be false to; to deceive; as, to betray a
      person or a cause.
      [1913 Webster]

            But when I rise, I shall find my legs betraying me.
                                                  --Johnson.
      [1913 Webster]

   3. To violate the confidence of, by disclosing a secret, or
      that which one is bound in honor not to make known.
      [1913 Webster]

            Willing to serve or betray any government for hire.
                                                  --Macaulay.
      [1913 Webster]

   4. To disclose or discover, as something which prudence would
      conceal; to reveal unintentionally.
      [1913 Webster]

            Be swift to hear, but cautious of your tongue, lest
            you betray your ignorance.            --T. Watts.
      [1913 Webster]

   5. To mislead; to expose to inconvenience not foreseen to
      lead into error or sin.
      [1913 Webster]

            Genius . . . often betrays itself into great errors.
                                                  --T. Watts.
      [1913 Webster]

   6. To lead astray, as a maiden; to seduce (as under promise
      of marriage) and then abandon.
      [1913 Webster]

   7. To show or to indicate; -- said of what is not obvious at
      first, or would otherwise be concealed.
      [1913 Webster]

            All the names in the country betray great antiquity.
                                                  --Bryant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

betray
    v 1: reveal unintentionally; "Her smile betrayed her true
         feelings" [syn: betray, bewray]
    2: deliver to an enemy by treachery; "Judas sold Jesus"; "The
       spy betrayed his country" [syn: betray, sell]
    3: disappoint, prove undependable to; abandon, forsake; "His
       sense of smell failed him this time"; "His strength finally
       failed him"; "His children failed him in the crisis" [syn:
       fail, betray]
    4: be sexually unfaithful to one's partner in marriage; "She
       cheats on her husband"; "Might her husband be wandering?"
       [syn: cheat on, cheat, cuckold, betray, wander]
    5: give away information about somebody; "He told on his
       classmate who had cheated on the exam" [syn: denounce,
       tell on, betray, give away, rat, grass, shit,
       shop, snitch, stag]
    6: cause someone to believe an untruth; "The insurance company
       deceived me when they told me they were covering my house"
       [syn: deceive, betray, lead astray] [ant: undeceive]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "betray":
   abuse, apostatize, babble, bamboozle, be indiscreet, be unguarded,
   bear witness against, beguile, betoken, betray a confidence, blab,
   blabber, blow the whistle, bluff, blurt, blurt out, bolt,
   break away, break faith, cajole, cheat on, circumvent, collaborate,
   conjure, cross, debauch, deceive, defect, defile, deflower, delude,
   demonstrate, desert, despoil, diddle, disclose, discover, divulge,
   double-cross, dupe, ensnare, entrap, evidence, evince, expose,
   fail, fink, fool, force, forestall, gammon, get around, give away,
   gull, hoax, hocus-pocus, hoodwink, hornswaggle, humbug, impart,
   indicate, inform, inform against, inform on, juggle, lay bare,
   lead astray, leak, let down, let drop, let fall, let slip,
   manifest, misguide, mislead, mock, narc, outmaneuver, outreach,
   outsmart, outwit, overreach, peach, pigeon, play one false,
   pull out, put something over, rape, rat, ravage, ravish, renegade,
   reveal, reveal a secret, ruin, run out on, secede, seduce, sell,
   sell out, shop, show, sing, snare, snitch, snitch on, snow, soil,
   spill, spill the beans, split, squeal, stool, string along, sully,
   take in, talk, tattle, tattle on, tell, tell on, tell secrets,
   tell tales, testify against, trap, trick, turn in, turn informer,
   two-time, uncover, unveil, violate