The Collaborative International Dictionary of English v.0.48:

Disdiapason \Dis*di`a*pa"son\, n. [Pref. dis- (Gr. ?) +
   diapason.] (Anc. Mus.)
   An interval of two octaves, or a fifteenth; -- called also
   bisdiapason. Compare diapason[1].
   [1913 Webster]