| In mining towns in West Virginia, mill towns in North Carolina, and sharecropping plantations across the South* people were regularly paid less than their cost of living and/or paid in "scrip" redeemable only at the company store. As a consequence, they went deeply into debt, (usually to their employer) and because of that debt they were prevented (often by force of law) from leaving their jobs or leaving town, and they could not take any legal action if their employer did something they didn't like. Like, say, beating them.
So let's see, you can't leave, your employer owns your home and most of your possessions, and your employer can basically do anything they want to you, including forcing your children to come work for him, and killing you with impunity. (I'll write up a node on Gastonia soon.) Sounds pretty close to most definitions I've heard of slavery.
* Not to mention modern day examples like sweatshops and migrant farm labor | 
