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]