Slavery is a where people are treated as property to buy and sell, and are forced to work.
Slaves can be held against their will since their capture, purchase or birth, and deprived of the right to leave, to refuse to work, or to demand compensation. Historically, slavery was recognized by many sociaties; recently times slavery has been outlawed in most sociaties but continues through the practices of debt bondage, indentured servitude, serfdom, domestic servants kept in captivity, certain adoptions in which children are forced to work as slaves, child soldiers, and forced marriage.
Slavery predates written records and has existed in many cultures. The number of slaves today is higher than at any point in history, remaining as high as 12 million to 27 million, though this is probably the smallest proportion of the world's population in history. Most are debt slaves, mainly in South Asia, who are under debt bondage incurred by lenders, sometimes even for generations. Human trafficking is primarily used for purpose of forcing women and children into sex industries.
From about the nineteenth century, "slavery" has been very closely associated with enslavement of non-white, usually black, people by white people. There was no such association; enslavement of any group of people, typically prisoners in a war, and capturing individuals to become slaves was commonplace.
In pre-industrial societies, slaves and their work were economically extremely important. In modern mechanised societies, there is much less need for sheer massive manpower; Norbert Wiener wrote that "mechanical labor has most of the economic properties of slave labor, though ... it does not involve the direct demoralizing effects of human cruelty.