The Collaborative International Dictionary of English v.0.48:

Sudden \Sud"den\, adv.
   Suddenly; unexpectedly. [R.]
   [1913 Webster]

         Herbs of every leaf that sudden flowered. --Milton.
   [1913 Webster]

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.
      [1913 Webster]

            Sudden fear troubleth thee.           --Job xxii.
                                                  10.
      [1913 Webster]

   2. Hastly prepared or employed; quick; rapid.
      [1913 Webster]

            Never was such a sudden scholar made. --Shak.
      [1913 Webster]

            The apples of Asphaltis, appearing goodly to the
            sudden eye.                           --Milton.
      [1913 Webster]

   3. Hasty; violent; rash; precipitate. [Obs.] --Shak.
      [1913 Webster]

   Syn: Unexpected; unusual; abrupt; unlooked-for.
        [1913 Webster] -- Sud"den*ly, adv. -- Sud"den*ness,
        n.
        [1913 Webster]

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.
      [1913 Webster]

            How art thou lost! how on a sudden lost! --Milton.
      [1913 Webster]

            He withdrew his opposition all of a sudden.
                                                  --Thackeray.
      [1913 Webster]

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]