Magic is the supernatural that is affected by the mind. Magic is reality’s recognition of language and ideas as they affect the world.
All things have essences. An essence is more than what the thing is; essence is both what it is and is meant to be. Magic is not a thing. Things exist, you can see them and hear them and touch them, and all things have their essence. Magic is the word we use to talk about the truth of things, their essence.
When a dragon can shrug off most swords but a well-wrought blade can puncture its scaly hide, it’s not “because a wizard turned the sword magic,” it is a clash of concepts and beliefs. The dragon’s hide tells swords that they cannot pierce it, because they are mere sticks wielded by mortal men. And they cannot. But the blade crafted by a Master Smith knows this is a lie, for it is a Sword, and the essence of a Sword is to Kill. And it does. That is magic.
While you can intend to work magic, you won’t have control over its actual manifestation unless you know what you’re doing. For example, one could simply pluck at the strings of their harp with the intent of enticing a river to notice their plight — but the exact effect will be unknown, unless the singer had learned beforehand from some hermit that the river in question is a particular fan of heartfelt ballads.