It can get rather political, but the general definition of giftedness is to be in the top 5% on a good, standardised test of cognitive ability, such as the WISC or WJ-III. If you were at the top of any representative group of just 20 children of the same age, you were clearly gifted. In some schemes, a smaller number qualify as highly, exceptionally then profoundly gifted.

It's worth nothing that there's no rule that someone can't be both gifted and dyslexic, for example.