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 ]