The Collaborative International Dictionary of English v.0.48:

Undeniably \Un`de*ni"a*bly\, adv.
   In an undeniable manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undeniably
    adv 1: to an undeniable degree or in an undeniable manner; "she
           is undeniably the most gifted student in the class"

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "undeniably":
   actually, admittedly, assuredly, beyond doubt, beyond question,
   certainly, clearly, decidedly, demonstrably, doubtless,
   doubtlessly, for a certainty, for real, in all conscience,
   incontestably, indeed, indisputably, indubitably, irrefragably,
   irrefutably, manifestly, no doubt, noticeably, observably,
   obviously, out of question, patently, positively, quite, really,
   sensibly, seriously, truly, unambiguously, undoubtedly,
   unmistakably, unquestionably, verily, visibly, without doubt