The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

heir presumptive
    n 1: a person who expects to inherit but whose right can be
         defeated by the birth of a nearer relative [ant: heir
         apparent]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

HEIR PRESUMPTIVE. A presumptive heir is one who, in the present
circumstances, would be entitled to the inheritance, but whose rights may be
defeated by the contingency of some nearer heir being born. 2 B1 Com. 208.
In Louisiana, the presumptive heir is he who is the nearest relation of the
deceased, capable of inheriting. This quality is given to him before the
decease of the person from whom he is to inherit, as well as after the
opening of the succession, until he has accepted or renounced it. Civ. Code
of Lo. art. 876.