One of my pet peeves is the idea that a surplus in the labor market and unemployment are the same thing.
It's just not. The definition of "unemployment" varies across societies, but it is usually some variant of "not working but in the market for work". That's not a labor surplus. If a labor surplus exists, it is a subset of the unemployed, but it's not the definition of unemployment. There can be unemployment in a labor market that's in equilibrium.
Anyway - the point of my title is that the macro class I'm TAing is using Mankiw, I'm planning a lesson for my review session right now on the chapter on unemployment, and Mankiw defines unemployment as a labor surplus in that chapter. I have to choke it down because I can't contradict the textbook, have them fail a test, and then have them tell the professor that I taught them something different.
Oh well. Still - this is a really bad habit of textbooks.