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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WordNet (r) 3.0 (2006):

suddenness
    n 1: the quality of happening with headlong haste or without
         warning [syn: abruptness, precipitateness,
         precipitousness, precipitance, precipitancy,
         suddenness]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "suddenness":
   abruptness, alacrity, expeditiousness, feverishness, furiousness,
   haste, hastiness, hurriedness, impatience, impetuosity,
   impetuousness, impulsiveness, inexpectation, overhastiness,
   precipitance, precipitancy, precipitateness, precipitation,
   precipitatousness, precipitousness, promptness, quickness,
   rashness, recklessness, speed, swiftness, unanticipation,
   unexpectedness