WordNet (r) 3.0 (2006):

nonoccurrence
    n 1: absence by virtue of not occurring

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "nonoccurrence":
   absence, awayness, blank, deprivation, emptiness, lack, negation,
   negativeness, negativity, neverness, nihility, nonbeing, nonentity,
   nonexistence, nonpresence, nonreality, nonsubsistence, not-being,
   nothingness, nowhereness, nullity, subtraction, unactuality,
   unreality, vacancy, vacuity, vacuum, void, want