Source : Webster's Revised Unabridged Dictionary (1913)

Heptagonal \Hep*tag"o*nal\, a. [Cf. F. heptagonal.]
   Having seven angles or sides.

   {Heptagonal numbers} (Arith.), the numbers of the series 1,
      7, 18, 34, 55, etc., being figurate numbers formed by
      adding successively the terms of the arithmetical series
      1, 6, 11, 16, 21, etc.