Yeah, it's impossible to define it. But thankfully, you don't have to unambiguously define ethics and integrity for all of humanity and posterity to get somewhere useful. We're not writing probably correct algorithms here, we're dealing with humans. Some linear combination of "something illegal", "something you don't want landing on the New York times next to your name", "I know it when I see it", and "if this still isn't clear for you, you test me at your own risk" would be sufficient to take care of it: either they'll figure it out by themselves or they'll find out the hard way.