The Collaborative International Dictionary of English v.0.48:

Sudden \Sud"den\, adv.
   Suddenly; unexpectedly. [R.]
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         Herbs of every leaf that sudden flowered. --Milton.
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The Collaborative International Dictionary of English v.0.48:

Sudden \Sud"den\, a. [OE. sodian, sodein, OF. sodain, sudain, F.
   soudain, L. subitaneus, fr. subitus sudden, that has come
   unexpectedly, p. p. of subire to come on, to steal upon; sub
   under, secretly + ire to go. See Issue, and cf.
   Subitaneous.]
   1. Happening without previous notice or with very brief
      notice; coming unexpectedly, or without the common
      preparation; immediate; instant; speedy. "O sudden wo!"
      --Chaucer. "For fear of sudden death." --Shak.
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            Sudden fear troubleth thee.           --Job xxii.
                                                  10.
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   2. Hastly prepared or employed; quick; rapid.
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            Never was such a sudden scholar made. --Shak.
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            The apples of Asphaltis, appearing goodly to the
            sudden eye.                           --Milton.
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   3. Hasty; violent; rash; precipitate. [Obs.] --Shak.
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   Syn: Unexpected; unusual; abrupt; unlooked-for.
        [1913 Webster] -- Sud"den*ly, adv. -- Sud"den*ness,
        n.
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The Collaborative International Dictionary of English v.0.48:

Sudden \Sud"den\, n.
   An unexpected occurrence; a surprise.
   [1913 Webster]

   All of a sudden, On a sudden, Of a sudden, sooner than
      was expected; without the usual preparation; suddenly.
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            How art thou lost! how on a sudden lost! --Milton.
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            He withdrew his opposition all of a sudden.
                                                  --Thackeray.
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WordNet (r) 3.0 (2006):

sudden
    adj 1: happening without warning or in a short space of time; "a
           sudden storm"; "a sudden decision"; "a sudden cure" [ant:
           gradual]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "sudden":
   aback, abrupt, abruptly, accelerated, breakneck, breathless, brisk,
   electrifying, expeditious, extraordinary, fast, fleet, forthwith,
   hasty, headlong, hurried, immediate, impatient, impetuous,
   improbable, impulsive, more than expected, nerve-shattering,
   out-of-the-way, overhasty, panting, past expectation, precipitant,
   precipitate, precipitous, quick, rapid, rash, reckless, rushing,
   shocking, short, speedy, startling, suddenly, surprising, swift,
   unannounced, unanticipated, unanticipatedly, unaware, undivined,
   unexpected, unexpectedly, unforeseeable, unforeseen, unguessed,
   unheralded, unhoped for, unlooked-for, unpredictable, unpredicted,
   unprepared for, unwonted, without warning