Source : Webster's Revised Unabridged Dictionary (1913)

Nonexistence \Non`ex*ist"ence\, n.
   1. Absence of existence; the negation of being; nonentity.
      --A. Baxter.

   2. A thing that has no existence. --Sir T. Browne.

Source : WordNet®

nonexistence
     n : the state of not existing [syn: {nonentity}] [ant: {being}]