adjective (a)

• beyond belief or understanding(adj.all)

  Example:
  At incredible speed.
  The book's plot is simply incredible.
  

  Synonym:
  unbelievable
  Antonym:
  believable, credible
  

  source: wordnet30
• Not credible; surpassing belief; too extraordinary and improbable to admit of belief; unlikely; marvelous; fabulous.(adjective)

  source: webster1913