Slavery is the systematic exploitation of labour. As a social-economic system, slavery is a legal or informal institution under which a person (called "a slave") is compelled to work for another (sometimes called "the master" or "slave owner").Evidence of slavery predates written records, and has existed to varying extents, forms and periods in almost all cultures and continents.Slaves are held against their will from the time of their capture, purchase, or birth, and are deprived of the right to leave, to refuse to work, or to receive compensation (such as wages) in return for their labour. As such, slavery is one form of unfree labour. Today, slavery is formally outlawed in nearly all countries, but the phenomenon continues to exist in various forms around the world. Today freedom from slavery is an internationally recognised human right. Article 4 of the Universal Declaration of Human Rights states: No one shall be held in slavery or servitude; slavery and the slave trade shall be prohibited in all their forms.