What is unemployment?

[LifeIsAPoissonProcess]

The definition in cracking the AP gives the definition "To be considered unemployed, a labor force participant must be willing and able to work, and must have made an effort to seek work in the past four weeks." How does this deal with underemployment and dissatisfied employees, wherein they meet the criteria above, but also have jobs? It seems odd that this is the only criteria for deeming someone "unemployed", and if this isn't the only criteria, why the book would not further specify. One might argue that half of the US is unemployed if this is the case.