bisdiapason

from 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]