If it isn't an specific body part or object, it's a "philia" meaning love of. Fetish is just used as a catch-all term to describe arousal by something that is not "normal" or normally involved in the sex act. As in:
Necrophilia- no need to describe THAT one.
Klismophilia = love of/sexual association with enemas
Algamatophilia= love of/sexual association with statues.
Harpaxophilia- sexual arousal from being robbed/burglarized.
Compared the the above, I'd say we are pretty normal.
Tickling fetish is referred to as knismolagnia.
http://www.sex-lexis.com/Sex-Dictionary/knismolagnia
I'm not sure why it's a lagnia instead of a philia- lust instead of love. I'm not a linguist by profession.
So yes, arousal from being tickled/tickling and/or an obsession with tickling would be classified as a fetish in layman's terms. I wouldn't worry too much about its classification, though. You enjoy it, it's harmless enough, and as you can see from my examples above, it's comparatively quite "normal". So have fun.