Source : Webster's Revised Unabridged Dictionary (1913)

Demonstrably \De*mon"stra*bly\, adv.
   In a demonstrable manner; incontrovertibly; clearly.

         Cases that demonstrably concerned the public cause.
                                                  --Clarendon.

Source : WordNet®

demonstrably
     adv : in an obvious and provable manner; "his documentary sources
           are demonstrably wrong" [syn: {provably}, {incontrovertibly}]