Underearners are those of us for various reasons who, for various reasons, are not living up to our earning potential. From what I have read, I would tag myself an underearner though not an extreme case. (Extreme cases are those with dramatic ups and downs around money, blame others for financial woes, give away services for free when they could get paid or think it is noble to be poor.)
Want to see where you are on the spectrum? Take this online quiz from Barbara Stanny, who wrote the book, Overcoming Underearning — A Five Step Plan to Richer Life.