adverb (r)

• definitely or positively (`sure' is sometimes used informally for `surely')(adv.all)

  Example:
  She certainly is a hard worker.
  

  Synonym:
  for certain, for sure, sure, sure as shooting, sure enough, surely
  

  source: wordnet30
• Without doubt or question; unquestionably.(adverb)

  source: webster1913