The Collaborative International Dictionary of English v.0.48:
Demonstrably \De*mon"stra*bly\, adv. In a demonstrable manner; incontrovertibly; clearly. [1913 Webster] Cases that demonstrably concerned the public cause. --Clarendon. [1913 Webster]WordNet (r) 3.0 (2006):
demonstrably adv 1: in an obvious and provable manner; "his documentary sources are demonstrably wrong" [syn: demonstrably, provably, incontrovertibly]