The Collaborative International Dictionary of English v.0.48:

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.
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            The mystic obelisks stand up
            Triangular, pyramidal.                --Mrs.
                                                  Browning.
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   2. (Crystallog.) Same as Tetragonal.
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   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]