noun (n)

• an evil supernatural being(noun.person)

  Synonym:
  daemon, daimon, devil, fiend
  

  source: wordnet30
• a cruel wicked and inhuman person(noun.person)

  Synonym:
  devil, fiend, monster, ogre
  

  source: wordnet30
• someone extremely diligent or skillful(noun.person)

  Example:
  He worked like a demon to finish the job on time.
  She's a demon at math.
  

  source: wordnet30
• A spirit, or immaterial being, holding a middle place between men and deities in pagan mythology.(noun)

  source: webster1913