'Heir apparent' definitions:

Definition of 'heir apparent'

noun

An heir whose right to an inheritance cannot be defeated if that person outlives the ancestor [ant: heir presumptive]

From: GCIDE

Heir \Heir\ ([^a]r), n. [OE. heir, eir, hair, OF. heir, eir, F. hoir, L. heres; of uncertain origin. Cf. Hereditary, Heritage.]
1. One who inherits, or is entitled to succeed to the possession of, any property after the death of its owner; one on whom the law bestows the title or property of another at the death of the latter. [1913 Webster]
I am my father's heir and only son. --Shak. [1913 Webster]
2. One who receives any endowment from an ancestor or relation; as, the heir of one's reputation or virtues. [1913 Webster]
And I his heir in misery alone. --Pope. [1913 Webster]
Heir apparent. (Law.) See under Apparent.
Heir at law, one who, after his ancector's death, has a right to inherit all his intestate estate. --Wharton (Law Dict.).
Heir presumptive, one who, if the ancestor should die immediately, would be his heir, but whose right to the inheritance may be defeated by the birth of a nearer relative, or by some other contingency. [1913 Webster]