Bear in mind that "slave" doesn't necessarily connote "chattel property" in all cultures at all times. Americans are accustomed to think of slavery that way because yes, African American slaves were indeed considered mere property, not human beings with rights, in the eyes of those states that allowed the practice. But in such cultures as ancient Egypt, Greece and Rome, slaves were generally considered persons and enjoyed varying, though limited, degrees of legal protection. So depending on the society and the time, there could be a thin line indeed between "indentured servitude" and "slavery."