PYRAMIDAL is a 9 letter word that starts with P.

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

2. \Py*ram"i*dal\, n. (Anat.)
   One of the carpal bones. See {Cuneiform}, n., 2
   (b) .