adverb (r)

• happening unexpectedly(adv.all)

  Example:
  Suddenly she felt a sharp pain in her side.
  

  source: wordnet30
• quickly and without warning(adv.all)

  Example:
  He stopped suddenly.
  

  Synonym:
  abruptly, dead, short
  

  source: wordnet30
• on impulse; without premeditation(adv.all)

  Example:
  He made up his mind suddenly.
  

  source: wordnet30