An opinion I hold which I think separates magic from science is that magic is inherently unknowable through science. It's effects and causes cannot be derived through known physics. That does not mean that magic doesn't have rules, far from it, simply that these rules are incompatible with scientific law on a very basic level. While their effects may be measurable according to scientific principles (a rock thrown with magic will still be stopped by a wall), but the essential things that make them up cannot.
This is interesting. I've never really thought deeply about this subject before, and I like this attempt to delineate between science and magic, but I have several quibbles with this definition, and would like to ask a few questions.
What does "Magic is inherently unknowable through science" actually mean to you, Fireborn? How do you limit "science" such that it can't study magic, assuming it is remotely regular and empirically observable? What does it mean to be deeply incompatible with scientific laws on a deep level when so-called "scientific laws" are simply the regularities observed in the universe as we have seen it so far?
It seems to me that a lot of people, including in this thread, somehow treat "Science" as a branch of knowledge fundamentally distinct from other ways of knowing things. It isn't. Science is a deeply human field, as well as a natural field, and it takes the world as it is given. Most of what really distinguishes science from other fields are the careful checks and balances on what needs to be done in order for something to be considered known, but does not limit the fields of study or what is actually possible, as long as it can be systematically repeated.
Well, I think of magic as being made of different stuff, such that it exists on a different level from matter that makes it direct observation impossible. It's only when you get large amounts of magic together and watch how it reacts with normal matter (say, through allomancy?) that you can really observe it all.
And to be clear, when I say science I mean natural laws like physics that have simply been observed by scientific processes. So, theoretically, magic could fit into science, it would simply exist under a different umbrella from other things.