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]