Definition of "septennial"

"Lawgivers were never more mistaken," said Lady Mary, "than when they ordained that the conjugal tie should last through life for better and worse; the last injunction being strictly complied with. There should be septennial marriages, as well as septennial parliaments!"

1837, L[etitia] E[lizabeth] L[andon], “An Act of Parliament”, in Ethel Churchill: Or, The Two Brides. […], volume II, London: Henry Colburn, […], page 189