Originally genius is a term from Roman mythology, see: genius (mythology). The modern meaning of the word is different; see below.
In general usage, a genius is either a polymath or a person gifted with particular talents, mainly regarding intelligence. Most people think of intellectual geniuses but there are social geniuses (for example Oscar Wilde) and athletic geniuses.
Term is reserved for those of extraordinary talent that goes far beyond what a non-genuis could ever hope to achieve. Geniuses often make huge original leaps in their field of work, rather than just extending previous work in that field.
Among persons widely called geniuses are:
Harold Bloom, Genius: A Mosaic of One Hundred Exemplary Creative Minds, Warner Books
James Gleick, Genius : The Life and Science of Richard Feynman, Vintage
Clifford A. Pickover, Strange Brains and Genius, QuillReferences