The Collaborative International Dictionary of English v.0.48:

Definitely \Def"i*nite*ly\, adv.
   In a definite manner; with precision; precisely;
   determinately.
   [1913 Webster]

WordNet (r) 3.0 (2006):

definitely
    adv 1: without question and beyond doubt; "it was decidedly too
           expensive"; "she told him off in spades"; "by all odds
           they should win" [syn: decidedly, unquestionably,
           emphatically, definitely, in spades, by all odds]

Moby Thesaurus II by Grady Ward, 1.0:

119 Moby Thesaurus words for "definitely":
   absolutely, and no mistake, apparently, apprehensibly,
   articulately, assuredly, at all events, at any rate, by all means,
   categorically, certainly, clearly, coherently, comprehensibly,
   concretely, conspicuously, dead, decidedly, decisively, directly,
   discernibly, distinctly, doubtless, doubtlessly, especially, even,
   evidently, exactly, explicitly, expressly, faithfully, finally,
   for a certainty, for a fact, for certain, for sure, forsooth,
   glaringly, in all respects, in detail, in every respect,
   in particular, in plain English, in plain terms, in plain words,
   in truth, indeed, indeedy, indubitably, intelligibly,
   ipsissimis verbis, item by item, just, limpidly, literally,
   literatim, lucidly, manifestly, markedly, minutely, most assuredly,
   most certainly, nothing else but, noticeably, observably,
   obviously, of course, particularly, patently, pellucidly,
   perceivably, perceptibly, perspicuously, plainly, plumb,
   point-blank, positively, precisely, prominently, pronouncedly,
   recognizably, right, rigidly, rigorously, seeably, separately,
   simply, singly, specially, specifically, square, squarely,
   staringly, starkly, straight, strictly, surely, to a certainty,
   to be specific, to be sure, to the letter, truly, unambiguously,
   unconcealedly, understandably, undeviatingly, undisguisedly,
   undoubtedly, unequivocally, unerringly, unmistakably,
   unquestionably, verbally, verbatim, verbatim et litteratim,
   visibly, with clarity, word by word, word for word