The Collaborative International Dictionary of English v.0.48:

Diametral \Di*am"e*tral\, a. [Gr. F. diam['e]tral.]
   Pertaining to a diameter; diametrical.
   [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diametral \Di*am"e*tral\, n.
   A diameter. [Obs.] --Sir T. Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diametral
    adj 1: related to or along a diameter; "the diametral plane"
           [syn: diametral, diametric, diametrical]