The Collaborative International Dictionary of English v.0.48:

Unprecedented \Un*prec"e*dent*ed\, a.
   Having no precedent or example; not preceded by a like case;
   not having the authority of prior example; novel; new;
   unexampled. -- Un*prec"e*dent*ed*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unprecedented
    adj 1: having no precedent; novel; "an unprecedented expansion
           in population and industry" [ant: precedented]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unprecedented":
   beguiling, bewildering, enigmatic, exceptional, extraordinary,
   fabulous, fantastic, fascinating, incomprehensible, inconceivable,
   incredible, indescribable, ineffable, legendary, marvelous,
   miraculous, mythical, noteworthy, outlandish, passing strange,
   phenomenal, portentous, primary, prodigious, prototypal, puzzling,
   rare, remarkable, sensational, strange, striking, stupefying,
   stupendous, uncopied, unduplicated, unexampled, unheard-of,
   unimaginable, unimitated, unique, unparalleled, unreproduced,
   unspeakable, wonderful, wondrous