Replacement child
In history it has long been a practice for an aristocratic wife to provide an "heir and a spare". That means an heir to continue the family line and a spare in case the heir died too young. In Royal families one spare was the minimum. When child mortality rates were high, more was considered even better to secure the throne. Queen Victoria had nine children. George III had thirteen. These children had different names, but the concept was and is the same. The difference between early heirs and spares and the modern practice is that today a female can inherit the throne.