Ŏc'to-dĕc'i-mal, a.

(-dĕs'i-mal), a. [Fr. octodécimal, Sp. octodecimal, fr. Lat. octodecim, eighteen, equivalent to decem et octo.] (Crystallog.) Having eighteen faces:—a term designating a crystal whose prism, or the middle part, has eight faces, and the two summits together ten faces.