Mind you, no-one called Leonardo a genius in his lifetime. Our modern use of the word seems to date from the 19th century- and from the work of Sir Francis Galton- another genius.
I suppose our use of the word has always been a bit loose. There are "universal" geniuses like Goethe- who excelled in all sorts of different disciplines and (according to a website I've just consulted) may have had the highest IQ of all time- and specialised geniuses like Einstein- who was great in a single field.
Exactly how brilliant do you have to be? I have no difficulty describing Mozart as a genius, but step down a few rungs and what about Tchaikovsky? What about Paul McCartney?
I reckon there ought to be a cut-off point. Below a certain level you're not a genius, just very talented.
Also there has to be achievement. Being extremely intelligent but doing nothing about it isn't good enough. And the achievement has to be important. Now that's another slippery word- but I reckon a significant breakthrough in philosophy, science or the arts puts you in the running, but being very good at chess doesn't.
I would hesitate to describe any sportsman or woman as a genius. Same goes (coming full circle) for people who design frocks.
Genius and saint are the highest titles we can give a human being. I think we should be as careful with the one as the Vatican is (or used to be before JPII got going) with the other. There are gifted people- and then there are people whose gift is so extraordinary as to seem almost supernatural: these are the geniuses- and there are very, very few of them.