The Collaborative International Dictionary of English v.0.48:

Nonexistence \Non`ex*ist"ence\, n.
   1. The state of not existing; absence of existence; the
      negation of being; -- usually used with reference to
      existince in the real world, or logical possiblity; as,
      the nonexistence of vaporware does not go unnoticed long.
      --A. Baxter.

   Syn: nonentity[1].
        [1913 Webster +PJC]

   2. A thing that has no existence. --Sir T. Browne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nonexistence
    n 1: the state of not existing [syn: nonexistence,
         nonentity] [ant: being, beingness, existence]