I'm so pleased this has sparked so much feed back!
My definition of cheating is not doing what you were tasked to do, and pretending you did.
If you don't do it, and you confess - then you failed but you didn't cheat. If you fail without a good reason... (and there can be good reasons, which I'm sure any good dom would understand and accept graciously) - but if you fail without a good reason... you have to be ready to take the consequences.
Like hellish and others, I find myself quite incapable of not telling him - well, everything. So I can't cheat, even when I want to. I can only fail, sometimes miserably - or, of course, succeed, sometimes wonderfully.
(Just an addendum - I absolutely do NOT consider what isabeau6 did to be cheating, because I do not consider that their partner at the time was a real dom. - the 'rules' didn't apply)