The Collaborative International Dictionary of English v.0.48:

Indubitable \In*du"bi*ta*ble\, a. [L. indubitabilis: cf. F.
   indubitable. See In- not, and Dubitable.]
   Not dubitable or doubtful; too evident to admit of doubt;
   unquestionable; evident; apparently certain; as, an
   indubitable conclusion. -- n. That which is indubitable.

   Syn: Unquestionable; evident; incontrovertible;
        incontestable; undeniable; irrefragable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indubitable
    adj 1: too obvious to be doubted [syn: beyond doubt(p),
           indubitable]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "indubitable":
   absolute, actual, admitting no question, apparent, bona fide,
   clear, clear as crystal, confirmable, crystal-clear, demonstrable,
   demonstratable, discernible, distinct, down, evident, explicit,
   express, factual, flat, genuine, historical, inappealable,
   incontestable, incontrovertible, indisputable, irrefragable,
   irrefutable, manifest, noticeable, observable, obvious,
   open-and-shut, palpable, patent, perceivable, perceptible,
   perspicuous, plain, plain as day, provable, real, seeable,
   self-evident, self-explaining, self-explanatory, sure-enough,
   tangible, testable, to be seen, true, unanswerable, unconfutable,
   undeniable, undoubted, unimpeachable, unmistakable, unquestionable,
   unrefutable, verifiable, veritable, visible