"market manipulation" in general is hard to define. The working definition in the US is something along the lines of "placing orders in the hopes that the price of the security will change in response to those orders existing, with no intention of actually executing the orders". There may be some specific regulations about specific types of market manipulation that are more clearly defined, but oftentimes not. There's lots of grey area, because the definition of market manipulation makes it seem like any order that's canceled instead of executed might be market manipulation. But in fact a majority of orders do get canceled before they trade!
So the real difference between market manipulation and a canceled order is just intention, so regulators have to make judgment calls sometimes.
> "market manipulation" in general is hard to define. The working definition in the US is something along the lines of "placing orders in the hopes that the price of the security will change in response to those orders existing, with no intention of actually executing the orders".
No, what you defined is "spoofing" - a much narrowed subcategory of market manipulation (which itself is gray, as you note). Market manipulation is broader and basically amounts to intentionally trying to affect the price of the security - even grayer.
So the real difference between market manipulation and a canceled order is just intention, so regulators have to make judgment calls sometimes.