Source : Webster's Revised Unabridged Dictionary (1913)

Heir \Heir\, 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.

            I am my father's heir and only son.   --Shak.

   2. One who receives any endowment from an ancestor or
      relation; as, the heir of one's reputation or virtues.

            And I his heir in misery alone.       --Pope.

   {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.

Source : WordNet®

heir presumptive
     n : a person who expects to inherit but whose right can be
         defeated by the birth of a nearer relative [ant: {heir
         apparent}]