The Collaborative International Dictionary of English v.0.48:

Preexist \Pre`["e]x*ist"\, v. i. [imp. & p. p. Pre["e]xisted;
   p. pr. & vb. n. Pre["e]xisting.]
   To exist previously; to exist before something else; to exist
   before the current activity commenced; as, preexisting
   illnesses are not covered by this insurance policy. [Also
   spelled pre-exist.]
   [1913 Webster + WordNet 1.5 + PJC]

WordNet (r) 3.0 (2006):

preexisting
    adj 1: existing previously or before something; "variations on
           pre-existent musical themes" [syn: preexistent, pre-
           existent, preexisting, pre-existing]