The Collaborative International Dictionary of English v.0.48:

Reversion \Re*ver"sion\ (r[-e]*v[~e]r"sh[u^]n), n. [F.
   r['e]version, L. reversio a turning back. See Revert.]
   1. The act of returning, or coming back; return. [Obs.]
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            After his reversion home, [he] was spoiled, also, of
            all that he brought with him.         --Foxe.
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   2. That which reverts or returns; residue. [Obs.]
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            The small reversion of this great navy which came
            home might be looked upon by religious eyes as
            relics.                               --Fuller.
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   3. (Law) The returning of an estate to the grantor or his
      heirs, by operation of law, after the grant has
      terminated; hence, the residue of an estate left in the
      proprietor or owner thereof, to take effect in possession,
      by operation of law, after the termination of a limited or
      less estate carved out of it and conveyed by him. --Kent.
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   4. Hence, a right to future possession or enjoyment;
      succession.
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            For even reversions are all begged before. --Dryden.
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   5. (Annuities) A payment which is not to be received, or a
      benefit which does not begin, until the happening of some
      event, as the death of a living person. --Brande & C.
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   6. (Biol.) A return towards some ancestral type or character;
      atavism.
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   Reversion of series (Alg.), the act of reverting a series.
      See To revert a series, under Revert, v. t.
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WordNet (r) 3.0 (2006):

reversion
    n 1: (law) an interest in an estate that reverts to the grantor
         (or his heirs) at the end of some period (e.g., the death
         of the grantee)
    2: (genetics) a return to a normal phenotype (usually resulting
       from a second mutation)
    3: a reappearance of an earlier characteristic [syn: atavism,
       reversion, throwback]
    4: turning in the opposite direction [syn: reversion,
       reverse, reversal, turnabout, turnaround]
    5: returning to a former state [syn: regression, regress,
       reversion, retrogression, retroversion]
    6: a failure to maintain a higher state [syn: backsliding,
       lapse, lapsing, relapse, relapsing, reversion,
       reverting]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "reversion":
   about-face, about-turn, atavism, back track, back trail, backing,
   backing off, backing out, backing up, backset, backsliding, backup,
   backward deviation, bequeathal, bequest, birthright,
   borough-English, coheirship, coparcenary, copyhold, devolution,
   ectropion, entail, equitable estate, estate at sufferance,
   estate for life, estate for years, estate in expectancy,
   estate in fee, estate in possession, estate tail, eversion,
   falling back, fee, fee simple, fee tail, feod, feodum, feud,
   feudal estate, fief, gavelkind, heirloom, heirship, hereditament,
   heritable, heritage, heritance, improvement,
   incorporeal hereditament, inheritance, instauration, introversion,
   intussusception, invagination, inversion, lapse, law of succession,
   lease, leasehold, legacy, legal estate, line of succession,
   mode of succession, paramount estate, particular estate, patrimony,
   postremogeniture, primogeniture, pronation, reactivation,
   reconstitution, reconversion, recrudescence, recurrence,
   redintegration, reenactment, reestablishment, reformation,
   regression, rehabilitation, reinstatement, reinstation,
   reinstitution, reinvestiture, reinvestment, relapse, remainder,
   renewal, replacement, restitution, restoration, resupination,
   retroflexion, retroversion, return, reversal, reverse, reversing,
   revulsion, right-about, right-about-face, setback, shifting trust,
   shifting use, succession, supination, swingaround, throwback,
   topsy-turviness, topsy-turvydom, transposal, transposition, turn,
   turnabout, turnaround, turning back, turning backwards,
   turning inside out, turning inward, turning over, ultimogeniture,
   vested estate, volte-face



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

REVERSION, estates. The residue of an estate left in the grantor, to
commence in possession after the determination of some particular estate
granted out by him; it is also defined to be the return of land to the
grantor, and his heirs, after the grant is over. Co. Litt. 142, b.
     2. The reversion arises by operation of law, and not by deed or will,
and it is a vested interest or estate, and in this it differs from a
remainder, which can never be limited unless by either deed or devise. 2 Bl.
Comm. 175; Cruise, Dig. tit. 17; Plowd. 151; 4 Kent, Comm. 349; 19 Vin. Ab.
217; 4 Com. Dig. 27; 7 Com. Dig. 289: 1 Bro. Civil Law, 213 Wood's Inst. 151
2 Lill. Ab. 483. A reversion is said to be an incorporeal hereditament. Vide
4 Kent, Com. 354. See, generally, 1 Hill. Ab. c. 52, p. 418; 2 Bouv. Inst.
n. 1850, et seq.