heir (n.) Look up heir at Dictionary.com
c.1300, from Anglo-French heir, Old French oir "heir, successor," from Latin heredem (nominative heres) "heir, heiress" (see heredity). Heir apparent (late 14c.) has the French order of noun-adjective, though it was not originally so written in English. It is the heir of one still alive whose right is clear. After death the heir apparent becomes the heir-at-law.