The Collaborative International Dictionary of English v.0.48:

Apparent \Ap*par"ent\, a. [F. apparent, L. apparens, -entis, p.
   pr. of apparere. See Appear.]
   1. Capable of being seen, or easily seen; open to view;
      visible to the eye; within sight or view.
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            The moon . . . apparent queen.        --Milton.
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   2. Clear or manifest to the understanding; plain; evident;
      obvious; known; palpable; indubitable.
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            It is apparent foul play.             --Shak.
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   3. Appearing to the eye or mind (distinguished from, but not
      necessarily opposed to, true or real); seeming; as the
      apparent motion or diameter of the sun.
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            To live on terms of civility, and even of apparent
            friendship.                           --Macaulay.
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            What Berkeley calls visible magnitude was by
            astronomers called apparent magnitude. --Reid.
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   Apparent horizon, the circle which in a level plain bounds
      our view, and is formed by the apparent meeting of the
      earth and heavens, as distinguished from the rational
      horizon.

   Apparent time. See Time.

   Heir apparent (Law), one whose to an estate is indefeasible
      if he survives the ancestor; -- in distinction from
      presumptive heir. See Presumptive.
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   Syn: Visible; distinct; plain; obvious; clear; certain;
        evident; manifest; indubitable; notorious.
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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.
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            I am my father's heir and only son.   --Shak.
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   2. One who receives any endowment from an ancestor or
      relation; as, the heir of one's reputation or virtues.
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            And I his heir in misery alone.       --Pope.
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   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.
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WordNet (r) 3.0 (2006):

heir apparent
    n 1: an heir whose right to an inheritance cannot be defeated if
         that person outlives the ancestor [ant: heir presumptive]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "heir apparent":
   Muslim rulers, Prinz, apparent heir, atheling, beneficiary heir,
   coparcener, crown prince, emir, fideicommissary heir,
   fiduciary heir, heir, heir expectant, heir general, heir in tail,
   heir of entail, heir of inventory, heir of line, heir portioner,
   heir presumptive, heir whatsoever, heiress, heritor, inheritor,
   inheritress, inheritrix, joint heir, khan, king, knez, mirza,
   next in line, presumptive heir, prince, prince consort,
   prince regent, princeling, princes of India, remainderman,
   reversioner, shahzada, sheikh, sherif, successor