The Collaborative International Dictionary of English v.0.48:

Demonstrable \De*mon"stra*ble\, a. [L. demonstrabilis: cf. OF.
   demonstrable, F. d['e]montrable.]
   1. Capable of being demonstrated; that can be proved beyond
      doubt or question.
      [1913 Webster]

            The grand articles of our belief are as demonstrable
            as geometry.                          --Glanvill.
      [1913 Webster]

   2. Proved; apparent. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

demonstrable
    adj 1: necessarily or demonstrably true; "demonstrable truths"
           [syn: demonstrable, incontrovertible]
    2: capable of being demonstrated or proved; "obvious lies"; "a
       demonstrable lack of concern for the general welfare";
       "practical truth provable to all men"- Walter Bagehot [syn:
       demonstrable, provable]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "demonstrable":
   absolute, actual, admitting no question, apodictic, apparent,
   attestable, authenticatable, certain, checkable, conclusive,
   confirmable, demonstratable, demonstrated, displayable, displayed,
   evident, evincible, exhibitable, exhibited, factual, historical,
   inappealable, incontestable, incontrovertible, indisputable,
   indubitable, irrefragable, irrefutable, manifest, obvious,
   positive, provable, real, self-evident, shown, substantiatable,
   supportable, sustainable, testable, unanswerable, unconfutable,
   undeniable, unimpeachable, unquestionable, unrefutable,
   validatable, verifiable