noun
1. : the structure of a mineral aggregate that is made up of nearly parallel slender columns and that is intermediate between an equant and acicular structure (as in some amphiboles)
2. : a geologic structure common in basalts and other lavas that is characterized by the division of the rock into more or less regular often vertical prisms or columns which ordinarily have six but may have from three to eight sides and which may be nearly vertical or nearly horizontal