The term “fetish” is used pretty loosely these days to mean anything that really really turns someone on.
But there is an official definition, and here it is…
“Sexual fetishism, or erotic fetishism, is the sexual arousal a person receives from a physical object, or from a specific situation”
So when the term is used “correctly” it is referencing a sexual need that goes far past desire into necessity. For someone to have a clinical fetish that person would have to have the fetishized item or situation involved 100% of the time in order to reach sexual arousal and climax.
What people frequently mean when they say fetish, is “kink”. Of course, language is fluid. Everyone works from a different internal dictionary and people might have a variety of experiences of words.
I’ve found that “kink” is a more accurate term for most. A good way to look at “having a kink” for something is that it’s an item/activity/situation that you find deeply, consistently erotic but is not necessary for your enjoyment of a sexual encounter.
Kink is also frequently used as an umbrella term for “the lifestyle” or as a sexual orientation. It can be used to describe a collection of activities (“kinky sex” might include some bondage or spanking, etc) that are enjoyed during sexual play.
Fantasies consist of the wide world of your erotic imagination. Having a fantasy about something does NOT mean you actually want it to happen in real life, which is one of the magical things about fantasies. In our minds we can explore experiences that may not even be possible, but are incredibly arousing to think about.
However using fantasies to fulfill a fetish (or kink!) can be a great compromise when the fetish (or kink!) is difficult or impossible to enact. For example, I’ve known many foot fetishists that fantasize about feet while having more traditional sex. This is helpful when one partner isn’t interested in the others fetish (or at least not every time) so that both can still have a happy sex life.
Bottom line, language and terminology are not worth getting hung up on. We fantasize about our fetishes and fetishize our fantasies. As long as we’re all getting (consensually!) what we need to feel sexually satisfied then that’s what’s important!