Relative is definitely the right word, as
@Jonathan touched on. If pressed, I think I would define abuse when I answer yes to one of these points:
1. Is this impacting other customers?
2. Should it be impacting other customers?
3. Does this usage limit capacity to levels that prevent minimum desired profit margins?
The problem here is that many customers always see the accusation as a statement of "I need to oversell this to such a degree that I can't have you bursting this resource in the way that you are, because I need maximum profit squeezed out of this server like water from a wet rag." They see it that way because, frankly, that is how a lot of the big names in the hosting industry have operated for years. You might not have that kind of intent to oversell, but there's still a number, and it's still relative. The customer doesn't really know what it's relative to though, and to some extent neither do you as the host.
You either sell dedicated resources or you take a risk that too many customers who neighbor each other will burst at the same time, or consistently use more than average together. If you're a good provider, you're looking at those customers and asking yourself "Should what they're doing be causing a problem for other customers? Is this because I took too big of a risk or because they're being unreasonable in their usage?" The answer requires context.
Example: Customer runs a blog. It's not popular. It's not featured anywhere popular. One IP is connecting to it. It's maxing out 6 CPU cores. Unreasonable.
Example 2: Customer is CPU mining bitcoin. Neighbors are seeing poor CPU/disk performance. Unreasonable.
Example 3: Customer runs a blog. It's just been featured on reddit. Thousands of IPs are connecting at once. It's maxing out 2 CPU cores. Perfectly reasonable.