Indentured servitude is a form of labor in which a person is contracted to work without salary for a specific number of years. The contract called an "indenture", may be entered voluntarily for a prepaid lump sum, as payment for some good or service, purported eventual compensation, or debt repayment. An indenture may also be imposed involuntarily as a judicial punishment. The practice has been compared to the similar institution of slavery, although there are differences.