The Collaborative International Dictionary of English v.0.48:

Evenness \E"ven*ness\, n.
   The state of being ven, level, or disturbed; smoothness;
   horizontal position; uniformity; impartiality; calmness;
   equanimity; appropriate place or level; as, evenness of
   surface, of a fluid at rest, of motion, of dealings, of
   temper, of condition.
   [1913 Webster]

         It had need be something extraordinary, that must
         warrant an ordinary person to rise higher than his own
         evenness.                                --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

evenness
    n 1: the parity of even numbers (divisible by two)
    2: a quality of uniformity and lack of variation [syn:
       evenness, invariability] [ant: unevenness,
       variability]
    3: the quality of being balanced [ant: unevenness]