well I can't tell you the definition, but I do know that people who have style are comfortable with themselves skiing. I notice that people who have the most style almost have a swagger to them when they ski. They have unnecessary movements that create a different visual from people who do the same trick.
A staple movement would be afterbang. you position yourself in a certain way, which is really unnecessary from an efficiency standpoint, to create a different way of landing something. Armterbang is another note-worthy visual movement. The smaller, more subtle things are the things that make people stand out in the majority of their skiing though. Things like excessive boot pressure when landing switch, getting a solid arch in your back when doing grabs like blunt, full body extension with backflips, etc.
I personally find style to be the comfortability level of someone skiing, but it is not a normal comfortability. I would say it is a kind that is comfortable with themselves too. I occasionally feel different when I ski on really nice days, and this different feeling reflects in my skiing. I am much more confident and flowyier(probably not a word) even if I have complete confidence in my skiing ability all the time. It is like a different part of yourself as a person and how comfortable you are with that part of yourself I guess would be a good analogy.
THats all I got. sorry if it makes zero sense