Heptagonal \Hep*tag"o*nal\, a. [Cf. F. heptagonal.]
Having seven angles or sides.
[1913 Webster]
{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.
[1913 Webster]