Indentured servitude was the practice of an individual being forced to perform work for another for a fixed period of time. As such, it counts as a form of slavery.
The Ferengi Commerce Authority had the authority to punish Ferengi females who earned profit, an illegal activity for them, by selling them into indentured servitude. Liquidator Brunt threatened to do that to Ishka if Quark did not get her to confess to her having earned a profit in 2371. (DS9: "Family Business")