The act of stans supporting their favorite idol or character. A form of otaku activities (ota-katsu), seen commonly among high school girls and young women. They can be distinguished from standard otaku as their nerdiness is centered on a single individual.
The word is used for someone being extremely dedicated to supporting their favorite - the object of their affection is their "oshi". (while "katsu" is short for katsudou - activities)
Most commonly shown through having tons of merchandise of a single idol or character, or through typical idol fan behavior. Jirai kei girls frequently practice oshi-katsu.
