Well, for one, it's not actually defined - and this is deliberate, because if one were try to define it in any meaningful way it could immediately be proven to be incorrect. For the most part, its proponents point to (somewhat arbitrary) statistics, but ignore counter-statistics (like incarceration rates of men vs. women, for example) that undermine their position.

You would say average income, education level, incarceration rate, etc. are arbitrary?

Is there any statistic where black people in America are performing better than any other race?