Magicka is everything that isn't happening right now, potential. For things to happen you need something for them to happen in, we'll call that void. When you put them together you get this grey mix in which random things are happening.
Now in this grey and turbulent maybe things start to be. It wasn't happening already so it was bound to happen because of all the potential things that could happen. These things, they didn't last. They're just messing around and suddenly they make themselves not-happen. Then one thing finds out how to make things happen in order. Again perfectly acceptable, hadn't happened yet.
So now we have allot of things imitating this one thing doing things in order and they can avoid not-happening or happening again right after the not-happening happened. Either way, about half these things happen to store things that could happen up. Big white chunks of possible events. Because they're all bunched up, there is no room for them to happen in. So they don't. The other half just makes everything happen, good, bad, chaotic, random anything goes, it's all way too much fun. So soon there is nothing to make happen any more. It's all just black and void.
Then one of these things decides he wants to see more. He's tired to light and dark so he comes up with a plan for something grey in which new things will always happen.
Sound familiar?
Exactly! You can't explain magic. It's make-believe stuff.
I just did! Magicka is that which makes things happen, that which makes change occur. It's what makes you believe.
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More seriously:
Crowley preferred the spelling magick, defining it as "the science and art of causing change to occur in conformity with the will." By this, he included "mundane" acts of will as well as ritual magic. In Magick in Theory and Practice, Chapter XIV, Crowley says:
What is a Magical Operation? It may be defined as any event in nature which is brought to pass by Will. We must not exclude potato-growing or banking from our definition. Let us take a very simple example of a Magical Act: that of a man blowing his nose. - http://en.wikipedia.org/wiki/Magick_%28Aleister_Crowley%29
forget about fantasy-plot-advancing-magic, there is room for philosophy.