Heir apparent
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
An heir whose right of inheritance is indefeasible, provided he outlive the ancestor; as in England the eldest son. or his issue, who must, by the course of the common law, be heir to the father whenever he happens to die.
2 Bl. Comm. 208; 1 Steph Comm. 358; Jones v. Fleming, 37 Hun (N. Y.) 230.