The Collaborative International Dictionary of English v.0.48:

Defiant \De*fi"ant\, a. [Cf. F. d['e]fiant, p. pr. of d['e]fier.
   See Defy.]
   Full of defiance; bold; insolent; as, a defiant spirit or
   act.
   [1913 Webster]

         In attitude stern and defiant.           --Longfellow.
   -- De*fi"ant*ly, adv. -- De*fi"ant*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

defiant
    adj 1: boldly resisting authority or an opposing force; "brought
           up to be aggressive and defiant"; "a defiant attitude"
           [syn: defiant, noncompliant] [ant: compliant]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "defiant":
   aggressive, antagonistic, arrogant, audacious, belligerent,
   beyond control, bold, brash, brassy, brazen, breachy, bumptious,
   challenging, cheeky, cocky, contemptuous, contumacious, daring,
   defying, derisive, disdainful, disobedient, disregardful,
   fractious, greatly daring, gutsy, headstrong, hostile,
   impatient of control, impertinent, impudent, incorrigible,
   indocile, indomitable, insolent, insubordinate, insuppressible,
   intractable, irrepressible, mutinous, obstinate, obstreperous,
   ornery, out of hand, pert, pugnacious, rebellious, recalcitrant,
   refractory, regardless of consequences, resistant, resisting,
   restive, saucy, self-willed, shrewish, spunky, stubborn,
   unbiddable, uncontrollable, ungovernable, unmalleable,
   unmanageable, unmoldable, unruly, unsubmissive, unyielding, wild