Having a proclivity for something in a sexual way- especially something not inherently sexual. Examples: latex, rubber, spanking, specific smell/odor, dangerous situations, etc. A fetish can often heighten sexual satisfaction or even become obsessive and needed to reach sexual satisfaction. A good fetish is like a good woman: once you find one, you will quickly tire of it/her.
The fetish of one person for people with amputations was as sick as anything I had ever heard.
An adult in a diaper and sucking on a pacifier is a real turn-on fetish for me.