adverb (r)

• in a manner or to a degree that could not be doubted(adv.all)

  Example:
  It was immediately and indubitably apparent that I had interrupted a scene of lovers.
  

  Synonym:
  beyond a doubt, beyond a shadow of a doubt, beyond doubt
  

  source: wordnet30
• Undoubtedly; unquestionably; in a manner to remove all doubt.(adverb)

  source: webster1913