: an object or bodily part whose real or fantasized presence is psychologically necessary for sexual gratification and that is an object of fixation to the extent that it may interfere with complete sexual expression
However, not all dictionaries use this definition. It appears to have a different meaning in its common use, than in its medical use. I can see where that would get very confusing. Plus, this gets circular. If we use this definition, then, by definition, a fetish would be necessary for arousal.
Hmph, very annoying.