The Collaborative International Dictionary of English v.0.48:

Nonexistent \Non`ex*ist"ent\, a.
   Not having existence.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonexistent
    adj 1: not having existence or being or actuality; "chimeras are
           nonexistent" [ant: existent, existing]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "nonexistent":
   absconded, absent, away, deleted, departed, devoid, disappeared,
   existless, extinct, fancied, fictive, gone, gone away, illusory,
   imaginary, imaginational, imagined, lacking, lost, lost to sight,
   lost to view, minus, missing, negative, no longer present, no more,
   nonattendant, not found, not present, notional, null, omitted,
   out of sight, past and gone, subtracted, supposititious,
   taken away, unactual, unexisting, unreal, vacuous, vanished,
   visional, void, wanting, without being