There are clinical and non-clinical definitions for what a fetish is. The clinical definition, the one doctor’s learn in school and use to diagnose people, sounds like this:
The paraphilic focus in Fetishism involves the use of nonliving objects (the "fetish"). Among the more common fetish objects are women's underpants, bras, stockings, shoes, boots, or other wearing apparel. The person with Fetishism frequently masturbates while holding, rubbing, or smelling the fetish object or may ask the sexual partner to wear the object during their sexual encounters. Usually the fetish is required or strongly preferred for sexual excitement, and in its absence there may be erectile dysfunction in males…Usually the Paraphilia begins by adolescence, although the fetish may have been endowed with special significance earlier in childhood. Once established, Fetishism tends to be chronic.
Fetish has also come to be used to denote any preference that has an erotic or sensual tinge to it, even if it isn’t part of their sex life. So someone who really loves shoes may describe themselves as having a shoe fetish. On the other hand, someone who likes to incorporate fur into their sex play may say they have fur fetish.
There are a variety of theories about what causes fetishes, but no agreement or “evidence” that points to a single explanation. Some people feel that they have cultivated a fetish while others identify a specific moment or time period in their past, often during childhood, when a fetish was created.
In and of themselves, most fetishes (as long as they are consensual and not causing harm) can be healthy sexual expressions. A fetish becomes a problem if it is causing you or your partner distress, or if it’s getting in the way of you living your life.