Voted "special snowflake".
I think, magic is a thing that should not be. An impossibility. Not something that breaks the natural laws of our, real world, but something that makes the people go "what the hell?! h... how?!"
Otherwise, that's just an unusual or rare skill. Think about this way: I absolutely can't play violin, but I understand that some people can — when someone plays La Follia, it is captivating, but it doesn't feel like sorcery. It's just something
I can't do. But
a weird rig that produces music from a mushroom? That's freaking witchcraft, something that I just can't comprehend, at least, at this point in time.
So, what can or cannot be classified as magic entirely depends on the perspective.
Now, from a more practical standpoint, when it comes to fantastic worlds of swords, sorcery and graverobbery, where I draw the line is reliability and understanding. Magic is a power beyond any mortal mind, it ain't no tool, and, more importantly, it can
never be no tool. Otherwise? That's just technology.
What's the difference between slinging firebolts and shooting a blaster? There's none. You do some series of manipulations and reliably get a projectile of superheated gas. Yeah, one is explained through plasma unobtanium mumbo-jumbo, the other through weaving Mystra mumbo-jumbo, but at the end of the day, both are just tools to inflict violence on people.
Now, imagine, that instead of just waving your hands and speaking weird words, you were calling forth a fire spirit? You ain't controlling the mofo, you just unleash it. That ain't no tool, and only a suicidal nutjob would call it a tool. That's a dangerous gamble, at best.