Source : Webster's Revised Unabridged Dictionary (1913)

Plainness \Plain"ness\, n.
   The quality or state of being plain.

Source : WordNet®

plainness
     n 1: the state of being unmixed with other material; "the
          plainness of vanilla ice cream"
     2: clarity as a consequence of being perspicuous [syn: {perspicuity},
         {perspicuousness}]
     3: the appearance of being plain and unpretentious
     4: an appearance that is not attractive or beautiful; "fine
        clothes could not conceal the girl's homeliness" [syn: {homeliness}]