Slavery is the status of being owned by another person.
Slaves were common in medieval times, but the practice of slavery was somewhat different from the chattel slavery practiced in North America from the 16th to 19th centuries; Medieval slavery was less institutionalized, and less likely to involve large numbers of enslaved people toiling for a single owner. Significant regional differences would have impacted the practice of slavery, and it is important to note that serfs were not slaves.
Slavery is illegal in the modern period.
Slavery in the SCA
Slavery is one of those aspects of medieval life that cannot be reenacted in the SCA, however well-intentioned the reenactors are. Peers would do well to remember that Squires and Apprentices are not slaves, and should not be treated as such.