1. Of or pertaining to a pyramid; in the form of a a pyramid; pyramidical; as, pyramidal cleavage. The mystic obelisks stand up Triangular, pyramidal. Mrs. Browning. 2. (Crystallog.) Same as Tetragonal. Pyramidal numbers (Math.), certain series of figurate numbers expressing the number of balls or points that may be arranged in the form of pyramids. Thus 1...
