The Collaborative International Dictionary of English v.0.48:

Preternatural \Pre`ter*nat"u*ral\ (?; 135), a. [Pref. preter +
   natural.]
   Beyond of different from what is natural, or according to the
   regular course of things, but not clearly supernatural or
   miraculous; strange; inexplicable; extraordinary; uncommon;
   irregular; abnormal; as, a preternatural appearance; a
   preternatural stillness; a preternatural presentation (in
   childbirth) or labor.
   [1913 Webster]

         This vile and preternatural temper of mind. --South.
   [1913 Webster]

   Syn: See Supernatural.
        [1913 Webster]

WordNet (r) 3.0 (2006):

preternatural
    adj 1: surpassing the ordinary or normal; "Beyond his
           preternatural affability there is some acid and some
           steel" - George Will; "his uncanny sense of direction"
           [syn: preternatural, uncanny]
    2: existing outside of or not in accordance with nature; "find
       transcendental motives for sublunary action"-Aldous Huxley
       [syn: nonnatural, otherworldly, preternatural,
       transcendental]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "preternatural":
   aberrant, anomalous, arcane, atypical, deviant, deviative, eerie,
   esoteric, extramundane, extraterrestrial, fey, heteroclite,
   hypernormal, hyperphysical, miraculous, mysterious, nonnatural,
   numinous, occult, otherworldly, preterhuman, preternormal,
   pretersensual, psychic, spiritual, superhuman, superior,
   supermundane, supernatural, supernormal, superphysical,
   supersensible, supersensual, supramundane, supranatural,
   transcendental, transmundane, unearthly, unhuman, unnatural,
   unrepresentative, untypical, unworldly