the state of being the firstborn of the same parents; seniority by birth among children of the same family.-the exclusive right of inheritance which belongs to the eldest son. thus in england the right of inheriting the estate of the father belongs to the eldest son, and in the royal family the eldest son of the sovereign is entitled to the throne by primogeniture. in exceptional cases, among the female children, the crown descends by right of primogeniture to the eldest daughter only and her issue.