The Collaborative International Dictionary of English v.0.48:

Nullity \Nul"li*ty\ (n[u^]l"l[i^]*t[y^]), n.; pl. Nullities.
   [LL. nullitias, fr. L. nullus none: cf. F. nullit['e] . See
   Null.]
   [1913 Webster]
   1. The quality or state of being null; nothingness; want of
      efficacy or force.
      [1913 Webster]

   2. (Law) Nonexistence; as, a decree of nullity of marriage is
      a decree that no legal marriage exists.
      [1913 Webster]

   3. That which is null.
      [1913 Webster]

            Was it not absurd to say that the convention was
            supreme in the state, and yet a nullity? --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nullity
    n 1: the state of nonexistence [syn: nothingness, void,
         nullity, nihility]
    2: something that is null (especially an enactment that has no
       legal validity)

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "nullity":
   a little thing, absence, aimlessness, cipher, dead letter,
   deprivation, dud, dummy, emptiness, empty sound, futility,
   hardly anything, hollow man, inanity, inessential, insignificance,
   insignificancy, jackstraw, lay figure, man of straw,
   marginal matter, matter of indifference, meaninglessness,
   mere noise, mere nothing, minor matter, nada, naught, nebbish,
   negation, negativeness, negativity, nihility, no great matter,
   nobody, noise, nonbeing, nonentity, nonexistence, nonoccurrence,
   nonreality, nonsensicality, nonsubsistence, not-being, nothing,
   nothing in particular, nothing to signify, nothingness,
   paltry affair, peu de chose, phatic communion, puppet,
   purposelessness, pushover, rien du tout, scarcely anything,
   senselessness, technicality, thing of naught, trifle, unactuality,
   unmeaningness, unreality, unsignificancy, vacancy, vacuity, vacuum,
   void, whiffet, whippersnapper, zero, zilch



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

NULLITY. Properly, that which does not exist; that which is not properly in
the nature of things. In a figurative sense, and in law, it means that which
has no more effect than if it did not exist, and also the defect which
prevents it from having such effect. That which is absolutely void.
     2. It is a yule of law that what is absolutely null produces no effects
whatever; as, if a man bad a wife in full life, and both aware of the fact,
he married another woman, such second marriage would be nun and without any
legal effect. Vide Chit, Contr. 228; 3 Chit. Pr. 522; 2 Archb. Pr. K. B. 4th
edit. 888; Bayl. Ch. Pr. 97.
     3. Nullities have been divided into absolute and relative. Absolute
nullities are those which may be insisted upon by any one having an interest
in rendering the act, deed or writing null, even by the public authorities,
as a second marriage while the former was in full force. Everything
fraudulent is null and void. Relative nullities can be invoked only by those
in whose favor the law has been established, land, in fact, such power is
less a nullity of the act than a faculty which one or more persons have to
oppose the validity of the act.
     4. The principal causes of nullities are,
     1. Defect of form; as, for example, when the law requires that a will
of land shall be attested by three witnesses, and it is on] attested by two.
Vide Will.
     5.-2. Want of will; as, if a man be compelled to execute a bond by
duress, it is null and void. Vide Duress.
     6.-3. The incapacities of the parties; as in the cases of persons non
compos mentis, of married women's contracts, and the like.
     7.-4. The want of consideration in simple contracts; as a verbal
promise with out consideration.
     8.-5. The want of recording, when the law requires that the matter
should be recorded; as, in the case of judgments.
     9.-6. Defect of power in the party who entered into a contract in
behalf of another; as, when an attorney for a special purpose makes an
agreement for his principal in relation to another thing. Vide Attorney;
Authority.
   10.-7. The loss of a thing which is the subject of a contract; as, when
A sells B horse, both supposing him to be alive, when in fact he was dead.
Vide Contract; Sale.
     Vide Perrin, Traite des Nullites; Henrion, Pouvoir Municipal, liv. 2,
c. 18; Merl. Rep. h.t.; Dall. Diet. h.t. See art. Void.