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):

of a sudden
    adv 1: happening unexpectedly; "suddenly she felt a sharp pain
           in her side" [syn: suddenly, all of a sudden, of a
           sudden]