I propose this definition: A genius is someone who, through their work, insipres the masses to take part in their field.
Because lets face it, your not a genius until everyone agrees you are a genius. Therefore, in order to be a genius you must make affect the masses.
In one of the Feynman books, a colleague said something like: With most really smart people, when they explain something to you, you get can it and think to yourself "I could of thought of that, if I was smarter." But with Feynman's work, even after you understood it, you'd think "How in the world did he come up with this?!"
"There are two kinds of geniuses: the 'ordinary' and the 'magicians'. An ordinary genius is a fellow whom you and I would be just as good as, if we were only many times better. There is no mystery as to how his mind works. Once we understand what they've done, we feel certain that we, too, could have done it. It is different with the magicians. Even after we understand what they have done it is completely dark. Richard Feynman is a magician of the highest calibre."