The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Syn: See Factitious.
        [1913 Webster] -- Un*nat"u*ral*ly, adv. --
        Un*nat"u*ral*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

unnaturalness
    n 1: the quality of being unnatural or not based on natural
         principles [ant: naturalness]