I've noticed over the past month or two that a ton of items have had their definitions changed. Into the future, this is probably a good thing, as the definitions are probably improving with the extra scrutiny, but it makes it hard on the end-user, especially with Hidden Pinyin turned on.

Would it be possible to have a notification displayed the first time you study an item after the definition has changed in the server? Something simple like an italicized "New Definition" under the definition, or a notification at the top of the study box, or even an option allowing users to accept or reject definition changes in their own study queue, leaving the old definition as a custom definition if they choose to reject it.

Hmm. That would be troublesome to code up, given the way definitions are changed and the way you encounter them. I don't think we will do this. We only change definitions when the new ones are better, so almost everyone should use any new definition, even if it is confusing at first to switch.