DIAMETRAL is a 9 letter word that starts with D.

1. \Di*am"e*tral\, a. [Gr. F. diam['e]tral.]
   Pertaining to a diameter; diametrical.
   
   {Diametral curve}, {Diametral surface} (Geom.), any line or
      surface which bisects a system of parallel chords drawn in
      a curve or surface.
   
   {Diametral planes} (Crystal.), planes in which two of the
      axes lie.
   
   

2. \Di*am"e*tral\, n.
   A diameter. [Obs.] --Sir T. Browne.