The ability to solve a problem that you haven't encountered before and for which there is no 'how to' manual to which you can turn.
Being capable of original thought and rejecting 'received wisdom'.
Thomas Paine said it best, 'I don't quote - I think.' (To quote Thomas Paine - I know!) Something we all should try, at least once in a while.
Mere knowledge - which can be very useful - is not a measure of how clever someone is. It only demonstrates that they have a good memory.
Intelligence cannot be taught.