I have touched on this subject before, but it bears repeating.
The "G" word is the single most abused and degraded term in the whole English language. It gets bandied about like an old hooker. I feel sorry for this poor girl, and want to help restore - if not her virginity - at least her natural loveliness and place in polite society.
Genius, n., (JEE - nyus): Applied to a person, it refers to a singular creator, whose unique vision has changed our collective perception of reality; whose achievements cannot fully be accounted for by his/her upbringing, influences or surroundings. This person may or may not be recognized in his own time, but the perceived cumulative value of his work increases with the years, until, with hindsight, he stands out from his contemporaries like a tall tree in a field of shrubs. Such people are vanishingly rare.
Of course deciding or agreeing whether some person or other rises to this description is a judgment call, and that ongoing debate is kind of what makes the world go around. Usually the person LEAST qualified to make that call is the candidate himself, who typically is too involved with his work to even think about it.
Possessing a high IQ or being very very good at what you do does NOT constitute "Genius."
The word derives from "genie," mythical spirits like the one who came out of Aladdin's lamp, who could confer magical powers on lowly humans. The word was used early on to describe a certain spiritual quality held by a whole culture, e.g., "the genius of the Greeks." It started to be applied to individuals - I may be wrong about this - around the time of Beethoven - a 'singular creator' if there ever was one - (c. 1800), and with the rise of the romantic conception of "The Artist" (Lord Byron, Liszt, Goethe et al), it came into general use as an individual descriptor.