noun (n)

• the property of having two dimensions(noun.attribute)

  Synonym:
  planeness, two-dimensionality
  

  source: wordnet30
• a want of animation or brilliance(noun.communication)

  Example:
  The almost self-conscious flatness of Hemingway's style.
  

  source: wordnet30
• a deficiency in flavor(noun.cognition)

  Example:
  It needed lemon juice to sharpen the flatness of the dried lentils.
  

  source: wordnet30
• the property of having little or no contrast; lacking highlights or gloss(noun.attribute)

  Synonym:
  lusterlessness, lustrelessness, mat, matt, matte
  

  source: wordnet30
• inactivity; showing an unusual lack of energy(noun.attribute)

  Synonym:
  languor, lethargy, phlegm, sluggishness
  

  source: wordnet30
• The quality or state of being flat.(noun)

  source: webster1913