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From: Webster's Revised Unabridged Dictionary (1913)

 Py·ram·i·dal a.
 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, 4, 10, 20, 35, etc., are triangular pyramidal numbers; and 1, 5, 14, 30, 55, etc., are square pyramidal numbers.