What is Jus Trium Liberorum?

Legal Definition
The ius trium liberorum, meaning “the right of three children” in Latin, was a privilege rewarded to Roman citizens who had borne at least three children or freedmen who had borne at least four children. It was a direct result of the Lex Iulia and the Lex Papia Poppaea, bodies of legislation introduced by Augustus in 18 BCE and 9 CE, respectively. These bodies of legislation were conceived to grow the dwindling population of the Roman upper classes. The intent of the jus trium liberorum has caused scholars to interpret it as eugenic legislation. Men who had received the jus trium liberorum were excused from munera. Women with jus trium liberorum were no longer submitted to tutela mulierum and could receive inheritances otherwise bequest to their children. The public reaction to the jus trium liberorum was largely to find loopholes, however. The prospect of having a large family was still not appealing. A person who caught a citizen in violation in this law was entitled to a portion of the inheritance involved, creating a lucrative business for professional spies. The spies became so pervasive that the reward was reduced to a quarter of its previous size. As time went on the ius trium liberorum was granted to those by consuls as rewards for general good deeds, holding important professions or as personal favors, not just prolific propagation. Eventually the ius trium liberorum was repealed in 534 CE by Justinian.
-- Wikipedia
Legal Definition
In Roman law. A right or privilege allowed to the parent of three or more children. 2 Kent, Comm. 85; 2 Bl. Comm. 247. These privileges were an exemption from the trouble of guardianship, priority in bearing offices', and a treble proportion of corn. Adame, Bom. Ant. (Am. Ed.) 227.
-- Black's Law Dictionary
Legal Definition
The extraordinary rights and immunities of a father of three or more children. See 2 Bl. Comm. 247.
-- Ballentine's Law Dictionary