The Collaborative International Dictionary of English v.0.48:
Functionally \Func"tion*al*ly\, adv. In a functional manner; as regards normal or appropriate activity. [1913 Webster] The organ is said to be functionally disordered. --Lawrence. [1913 Webster]WordNet (r) 3.0 (2006):
functionally adv 1: with respect to function; "the two units are functionally interdependent"