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.

a law requiring that property go to the lawful heir(s), designated as (priority order): eldest son; if eldest son dies without issue; (s.p.), then to next eldest; if no sons, then to father of deceased if still living, then to next eldest brother of deceased, etc. Example. See also Lines of Descent

The right of the eldest son to inherit the estate or office of his father. (MEDIEV-L. Medieval Terms) System of inheritance by which the first-born son succeeds to all his father's landed property. (Frame, Robin. Colonial Ireland, 1169-1369, 144)