There is an interesting article in the current issue of Commentary Magazine, about the definition of genius, noting that the term is used pretty widely these days about many people, http://www.commentarymagazine.com/article/i-dream-of-genius/.
There was on particular quote I liked from Schopenhauer defining the different between genius and talent:
“Talent is like the marksman who hits a target, which others cannot reach,” wrote Schopenhauer. “Genius is like the marksman who hits a target, which others cannot see.”