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)