Slavery as most contemporary Americans and Europeans will understand it is 'Chattel slavery' - that is, the slave is the personal property of the master, like cattle are the property of a farmer.

But there have been (and continue to be) many different types of slavery, such as 'bonded labour' - which was like an alternative way of paying your debts if you lacked the funds.

There is also 'forced labour' - in which, the slave is forced through threat of violence, rather than being socially accepted as the property of the master.

And finally, there is forced marriage, although this can overlap with forced labour (especially in the case of sex trafficking), its continued prevalence as a cultural practice usually makes countries view it as a separate problem.

In my research on slavery, I've found there are three basic ways that most academics define what counts as slavery:

• Economic - Does a person 'own' his or her labor and its products? This definition of slavery largely arose out of Marx and his followers writing in the 1800s. In Marx's grand scheme of history, slavery was replaced by feudalism. Serfs were tied to the land and had to give some of their labor and produce to their lord, but they were not owned in the same way as Roman slaves had been.

• Legal - Is a person considered the property of another? This definition of slavery is very similar to the first, but whereas economic definitions of slavery tend to focus on the exploitation of labor, legal definitions focus more on rights. In the case of medieval peasants, economic historians might classify them as serfs, since they own their labor and its fruits, but legal historians might call those same peasants slaves, since they have no recourse to justice beyond their landlord. ("Serf" and "slave" both translate the same Latin word, servus.)

• Experiential - Does a person live like a slave? In the 1980s, an anthropologist named Orlando Patterson defined slavery as (1) being under the power of another, (2) being separated or separable from your family, and (3) lacking honor or dignity. This has been a profoundly influential definition of slavery, since it allows comparisons across time and space.

Each definition has its problems. The first two are in some ways too rigorous, since they look for very specific factors in the past, as seen in the example of their differing definitions of the medieval peasant as slave or serf. But a flexible definition like Patterson's makes it difficult for historians to decide who is and who isn't a slave. For example, Patterson's criteria might classify a person drafted for the Vietnam War as a slave, but does that tell us anything useful about either slavery or the Vietnam War? I'm not certain.

Perhaps even a bigger problem with Patterson's definition is that it requires the researcher to get into the head of another person, and most people who we'd like to describe as slaves have left absolutely no record about what they thought about power, their families, or their sense of honor. In the end, trying using an experiential definition like Patterson's tells us more about our opinions as researchers than it does about the people we're trying to study.

In my own work, I tend to look at practice - Are people actually doing things that look like slavery or slaving? There's two problems with this. First, I adopt a flexible definition of slavery based on the treacherous ground of common sense: Can I expect the average educated individual to consider the thing I'm talking about as an example of slavery? The second problem is that my approach is presentist. It's rooted firmly in what we think about slavery today, and it largely ignores whether people in the past actually thought about themselves as suffering from slavery.

Neither of these problems is fatal. I actually study human trafficking, so calling it slavery is a pretty safe bet. Most people will feel comfortable calling human trafficking an example of slavery, and the early medieval people victimized by these practices almost certainly saw themselves as victims of something, regardless of whether they could describe it according to a concept of slavery. And where there is gray area--like the practice of donating children to a church--I take pains to describe it as such. Particularity is a cornerstone of the historian's craft.

Credit where credit is due: The economic, legal, experiential classification comes from Alice Rio “Freedom and Unfreedom in Early Medieval Francia: The Evidence of the Legal Formulae,” Past and Present 193 (2006), 7-40.

Also important to consider is the difference between what is called "gang slavery" and "task slavery". This is specific to its development in the United States, however.

Task systems are particular to colonial America prior to the antebellum period and focuses on the completion of the work as a whole. After the work was done, slaves were relieved of duty for the day.

Gang slavery developed during the antebellum industrial period of American slavery. Gang slavery is much more brutal and requires constant supervision and driving. It's the typical depiction of slaves working while shackled together, all while the guard watches on horseback gun in hand.