The Collaborative International Dictionary of English v.0.48:

Incontrovertible \In*con`tro*ver"ti*ble\, a.
   Not controvertible; too clear or certain to admit of dispute;
   indisputable. --Sir T. Browne. --
   In*con`tro*ver"ti*ble*ness, n. -- In*con`tro*ver"ti*bly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incontrovertible
    adj 1: impossible to deny or disprove; "incontrovertible proof
           of the defendant's innocence"; "proof positive"; "an
           irrefutable argument" [syn: incontrovertible,
           irrefutable, positive]
    2: necessarily or demonstrably true; "demonstrable truths" [syn:
       demonstrable, incontrovertible]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "incontrovertible":
   absolute, actual, adducible, admissible, admitting no question,
   attestative, attestive, authentic, based on, certain,
   circumstantial, conclusive, confirmable, convincing, cumulative,
   damning, decisive, definite, definitive, demonstrable,
   demonstratable, determinative, documentary, documented,
   established, evidential, evidentiary, ex parte, eye-witness,
   factual, final, firsthand, founded on, grounded on, hearsay,
   historical, implicit, inappealable, incontestable, indicative,
   indisputable, indubitable, irrefragable, irrefutable, irresistible,
   material, nuncupative, overwhelming, positive, presumptive,
   probative, provable, real, reliable, self-evident, significant,
   suggestive, sure, symptomatic, telling, testable, unanswerable,
   unconfutable, uncontestable, undeniable, unequivocal,
   unimpeachable, unquestionable, unrefutable, valid, verifiable,
   weighty