The Collaborative International Dictionary of English v.0.48:

Assure \As*sure\, v. t. [imp. & p. p. Assured; p. pr. & vb. n.
   Assuring.] [OF. ase["u]rer, F. assurer, LL. assecurare; L.
   ad + securus secure, sure, certain. See Secure, Sure, and
   cf. Insure.]
   1. To make sure or certain; to render confident by a promise,
      declaration, or other evidence.
      [1913 Webster]

            His promise that thy seed shall bruise our foe . . .
            Assures me that the bitterness of death
            Is past, and we shall live.           --Milton.
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   2. To declare to, solemnly; to assert to (any one) with the
      design of inspiring belief or confidence.
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            I dare assure thee that no enemy
            Shall ever take alive the noble Brutus. --Shak.
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   3. To confirm; to make certain or secure.
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            And it shall be assured to him.       --Lev. xxvii.
                                                  19.
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            And hereby we know that we are of the truth, and
            shall assure our hearts before him.   --1 John iii.
                                                  19.
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   4. To affiance; to betroth. [Obs.] --Shak.
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   5. (Law) To insure; to covenant to indemnify for loss, or to
      pay a specified sum at death. See Insure.
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   Syn: To declare; aver; avouch; vouch; assert; asseverate;
        protest; persuade; convince.
        [1913 Webster]

WordNet (r) 3.0 (2006):

assure
    v 1: make certain of; "This nest egg will ensure a nice
         retirement for us"; "Preparation will guarantee success!"
         [syn: guarantee, ensure, insure, assure, secure]
    2: inform positively and with certainty and confidence; "I tell
       you that man is a crook!" [syn: assure, tell]
    3: assure somebody of the truth of something with the intention
       of giving the listener confidence; "I assured him that
       traveling to Cambodia was safe"
    4: be careful or certain to do something; make certain of
       something; "He verified that the valves were closed"; "See
       that the curtains are closed"; "control the quality of the
       product" [syn: see, check, insure, see to it,
       ensure, control, ascertain, assure]
    5: cause to feel sure; give reassurance to; "The airline tried
       to reassure the customers that the planes were safe" [syn:
       reassure, assure] [ant: vex, worry]
    6: make a promise or commitment [syn: promise, assure]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "assure":
   afford hope, and candle, ascertain, assert, assert under oath,
   asseverate, attest, augur well, avouch, back, be convincing,
   be sponsor for, bear up, bid fair, bolster, bond, book, brace up,
   bring home to, bring over, bring round, bring to reason, buck up,
   carry conviction, certify, cheer, cinch, clear up, clinch, comfort,
   condole with, confirm, console, convert, convict, convince,
   countersecure, countersign, decide, depone, depose, determine,
   dismiss all doubt, drive home to, ease, embolden, encourage,
   endorse, ensure, establish, find out, fix, get at, give comfort,
   give hope, guarantee, guaranty, have good prospects, hearten,
   hold out hope, hold out promise, inspire, inspire belief,
   inspire hope, inspirit, insure, justify hope, kiss the book,
   lead to believe, make a promise, make certain, make fair promise,
   make no doubt, make no mistake, make sure, make sure of, nail down,
   nerve, persuade, pledge, plight, promise, put at ease,
   raise expectations, raise hope, reassure, relieve,
   remove all doubt, satisfy, secure, see that, see to it, sell,
   sell one on, set at ease, set at rest, settle, sign, sign for,
   solace, sort out, sponsor, stabilize, stand behind, stand up for,
   state, subscribe to, support, swear, swear by bell,
   swear the truth, swear to, swear to God, swear to goodness,
   sympathize with, talk over, testify, troth, undersign, underwrite,
   vouch, vow, warrant, win over