Source : Webster's Revised Unabridged Dictionary (1913)

Unnatural \Un*nat"u*ral\ (?; 135), a.
   Not natural; contrary, or not conforming, to the order of
   nature; being without natural traits; as, unnatural crimes.

   Syn: See {Factitious}. -- {Un*nat"u*ral*ly}, adv. --
        {Un*nat"u*ral*ness}, n.

Source : WordNet®

unnaturalness
     n : the quality of being unnatural or not based on natural
         principles [ant: {naturalness}]