What is the meaning of flatness?

noun

• The property of having two dimensions.
  • synonyms: two-dimensionality, planeness
• A want of animation or brilliance.
  • usage: "the almost self-conscious flatness of Hemingway's style"
• A deficiency in flavor.
  • usage: "it needed lemon juice to sharpen the flatness of the dried lentils"
• The property of having little or no contrast; lacking highlights or gloss.
  • synonyms: lusterlessness, lustrelessness, mat, matt, matte
• Inactivity; showing an unusual lack of energy.
  • usage: "the general appearance of sluggishness alarmed his friends"
  • synonyms: languor, lethargy, sluggishness, phlegm