in Geometry, a figure consisting of seven sides and as many angles. In fortification, a place is termed a heptagon, that has seven bastions for its defence.

HEPTAGONAL numbers, in Arithmetic, a sort of polygonal numbers, wherein the difference of the terms of the corresponding arithmetical progression is 5. One of the properties of these numbers is, that if they be multiplied by 49, and 9 be added to the product, the sum will be a square number.