[syn: languor, lethargy, sluggishness, phlegm, flatness]
The Collaborative International Dictionary of English v.0.48:
flatness \flat"ness\, n.
   1. The quality or state of being flat.
      [1913 Webster]
   2. Eveness of surface; want of relief or prominence; the
      state of being plane or level.
      [1913 Webster]
   3. Want of vivacity or spirit; prostration; dejection;
      depression.
      [1913 Webster]
   4. Want of variety or flavor; dullness; insipidity.
      [1913 Webster]
   5. Depression of tone; the state of being below the true
      pitch; -- opposed to sharpness or acuteness.
      [1913 Webster]
WordNet (r) 3.0 (2006):
flatness
    n 1: the property of having two dimensions [syn: two-
         dimensionality, flatness, planeness]
    2: a want of animation or brilliance; "the almost self-conscious
       flatness of Hemingway's style"
    3: a deficiency in flavor; "it needed lemon juice to sharpen the
       flatness of the dried lentils"
    4: the property of having little or no contrast; lacking
       highlights or gloss [syn: flatness, lusterlessness,
       lustrelessness, mat, matt, matte]
    5: inactivity; showing an unusual lack of energy; "the general
       appearance of sluggishness alarmed his friends" [syn:
       languor, lethargy, sluggishness, phlegm, flatness]