The Collaborative International Dictionary
Orthopinacoid
Orthopinacoid \Or`tho*pin"a*coid\, n. [Ortho- + pinacoid.] (Crystallog.) A name given to the two planes in the monoclinic system which are parallel to the vertical and orthodiagonal axes.
Wiktionary
orthopinacoid
n. (context crystallography English) Either of the two planes in the monoclinic system which are parallel to the vertical and orthodiagonal axis.