As with the more standard definition of the word, some sort of malign altering influence that subverts and changes an entity or person, that in this case happens to be fetishised or euphemistic to some degree.

Corruption in a fetish context refers to a character experiencing or inflicting one or more of the following for the purpose of erotic gratification:
1. a physical transformation into, or adoption of features possessed by, a group or entity regarded (by society or the transforming character specifically) as repellant.
2. a psychological transformation resulting in a character's loss of morality, degradation of righteousness, or subversion of their innocence.
3. a psychological transformation that leads to a character's surrender, acceptance, embrace, or active pursuit of that which would otherwise be taboo/resisted.

Worthy of Websters, Vikki!

Thank you. : ) I rather liked Myuphrid and Djack's answers too.

However, I only get half-credit because I didn't provide my thoughts on the subject. Personally, I think it opens up all kinds of neat possibilities in roleplay, but I've only played with it in limited terms (e.g., being converted from an innocent, shy, inexperienced young vixen into an obedient, slavish rubber sextoy; being seduced by an older sister before I can be approached by a boy; being a defeated angel enslaved by a demon prince/princess which causes me to gradually lose my halo and take on tiny horns, wings turn leathery and batlike, eyes turn red, clothing turning from white silk to scale black rubber, etc.).