This is an excellent question!

One of the major problems with trying to do economic history in the Early Modern or Pre-Modern periods is distinguishing between various types of labor obligations and relationships. Historians have in the past fruitlessly spent many hours trying to define the difference between slavery and other "not slavery" types of labor relationships. But the distinction between slavery and "free" can be fuzzier in practice than one might expect. So current labor history tends to view chattel slavery, like the type practiced in the 18th century North America, as being only one type of "unfree labor" with other types of labor obligations like indentured servitude, and even apprenticeship contracts, as all being types of "unfree labor". Thus in some sense we can view labor relations as existing on a specturm, the most venial types of chattel slavery at one end, and the charitable labors of the independently wealthy at the other end, and various sorts of serfdom and even many types of waged labor, existing somewhere in the middle.

Thus the term "unfree labor" includes not only various types of chattel slavery (what most people think of when they say "slavery"), but a variety of other types of forced labor, including debt slavery, serfdom, corvee, and prison camps.

The above is summarized generously from Tom Brass's excellent overview Towards a Comparative Political Economy of Unfree Labour: Case Studies and Debates