The Latin libertus or libertinus (pl. liberti; libertini) means a slave who has been manumitted. Through manumission a slave was given his freedom. With his freedom a former slave became a freedman. If (1) his master was a Roman citizen, and (2) he manumitted his slave formally, and (3) the freedman was of age, then the freedman became a citizen; otherwise, he did not. The freedman's master became his patron. Upon gaining his freedom, the freedman, who formerly had only one slave name, gained the name of his master's family as praenomen and nomen. The slave name remained, as cognomen.
A slave might become a libertus by buying his freedom. It may seem surprising that a slave would be able to do so, but slaves were allowed to accumulate money, known as the slave's peculium.