2. (Science: chemistry) same as tetragonal.
(Science: mathematics) Pyramidal numbers, certain series of figurate numbers expressing the number of balls or points that may be arranged in the form of pyramids. Thus 1, 4, 10, 20, 35, etc, are trangular pyramidal numbers; and 1, 5, 14, 30, 55, etc, are square pyramidal numbers.
Origin: Cf. F. Pyramidal.