A definition that falsifies itself, given that the most important overriding rule of the game is that what the DM says, goes. Individual DMs may and do have their own distinctions about how and when they will exercise that authority, but there is no such thing as a DM overriding the rules themselves.
How is this falsified? My definition is in no ways counters what you said. Rule 0 is DM Fiat by definition. Again, the negative connotations that you associate with DM Fiat are your own, not part of the term itself. There's nothing inherently wrong with DM fiat.
When the DM decides that he or she doesn't like a particular rule, and then changes that rule, that's not necessarily fiat because it's not necessarily being done to produce a specific result. If I think jumping is too easy, so, I jack up the jump DC, that's not fiat, necessarily. All I'm doing is changing the odds.
OTOH, if I change the DC to the point where I know that your character cannot succeed, or will automatically succeed, then that's fiat. I've changed the rules to the point where I am getting a specific result.
To recap - changing the rules isn't necessarily DM fiat. Changing the rules to produce a specific result? Yup, that's fiat. It's all about the results. Choosing different models for abstracting an action or object within the game world? Not fiat. Changing the model to the point where there can only be one result? That's fiat.
Another example. In 1e D&D there were no rules for long distance jumping. So, different DM's would give different rulings - Str or Dex check, possibly a save vs Paralysis, that sort of thing. That's not fiat, since all they've done is choose a particular method for modelling jumping. However, once you've picked that method, if you then start futzing about with it so that a PC cannot succeed because you don't think it's possible for him to succeed ("You can't jump in plate mail" is a pretty common occurrence at many tables), now you're into fiat territory.
And, finally, it's not like there's a hard and fast definition here. There are shades of grey.
However, at the end of things, fiat is, in itself, neither positive nor negative, it's simply another tool in the DM's kit.