Diametral \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.