Call me a nerd, but here's a psych definition from the DSM V,
"Fetishism was typically operationally
described as persistent preferential sexual arousal in association
with non-living objects, an over-inclusive focus on (typically
non-sexual) body parts (e.g., feet, hands) and body secretions. In
the DSM-III-R, Partialism, an ‘‘exclusive focus on part of the
body,’’was cleaved from Fetishism and added to the Paraphilia
Not Otherwise Specified category. The current literature reviewed
suggests that Partialism and Fetishism are related, can
be co-associated, and are non-exclusive domains of sexual behavior."
Essentially it can be a fixation or obsession for some, while for others it's something they find arousing. I feel like the definition is pretty fluid.
Oh, and here's the source for the above excerpt: link