I'm confused -- what was the old definition of "slave" which was so different from "person forced to work and prevented by force from leaving"?

I'm not convinced that there are indeed 27 million people fitting that definition at the moment, but I can't see how someone fitting that description is anything other than a slave. (Hmm, though I suppose legitimate prisoners can be forced to do work without being "slaves" so perhaps the definition does require tinkering around the edges.)