Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

preexist
     v : exist beforehand or prior to a certain point in time; "Did
         this condition pre-exist?"