Slavery is a system in which a person, a "slave", is bought and sold as property.
In the days of the original Systems Commonwealth the practice of slavery was outlawed and the High Guard enforced this law whenever possible. However, after the First Systems Commonwealth Civil War, slavery became a major industry in Known Space. Many of the Nietzschean Prides either own slaves or sell slaves. These worlds, known as "slave worlds", provide resources and manual labor to their Nietzschean masters, the Earth being an example.