The Collaborative International Dictionary of English v.0.48:

Diadelphian \Di`a*del"phi*an\, Diadelphous \Di`a*del"phous\, a.
   [Cf. F. diadelphe.] (Bot.)
   Of or pertaining to the class Diadelphia; having the stamens
   united into two bodies by their filaments (said of a plant or
   flower); grouped into two bundles or sets by coalescence of
   the filaments (said of stamens).
   [1913 Webster]