Pyramidal \Py*ram`i*dal\, a. [Cf. F. pyramidal.]

[1913 Webster]

1. Of or pertaining to a pyramid; in the form of a a pyramid; pyramidical; as, pyramidal cleavage. [1913 Webster]

The mystic obelisks stand up Triangular, pyramidal. --Mrs. Browning. [1913 Webster]

2. (Crystallog.) Same as Tetragonal. [1913 Webster]

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, 4, 10, 20, 35, etc., are triangular pyramidal numbers; and 1, 5, 14, 30, 55, etc., are square pyramidal numbers. [1913 Webster]

Source: The Collaborative International Dictionary of English v.0.48
 

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, 4, 10, 20, 35, etc., are triangular pyramidal numbers; and 1, 5, 14, 30, 55, etc., are square pyramidal numbers.

Source: Webster's Revised Unabridged Dictionary (1913)