HEPTAG'ONAL, adjective Having seven angles or sides. 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 are multiplied by 40, and 9 is added to the product, the sum will be a square number.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI