lex regia

• rēgia lex

Literally, “(the) royal law”. The process by which the term came to refer to the definitive transfer of authority from the Roman people to the emperor is obscure and controversial: although Ulpian (c. 220 CE) uses the term in describing the emergence of the principate, it is unclear whether he intended it specifically or in the generic sense of a royal law; it has also been argued that its appearance in Ulpian is a later Byzantine interpolation. The particular sense was established by the time of the Corpus Juris Civilis of Justinian (529–34).[1]

lex rēgia f (genitive lēgis rēgiae); third declension (law)

1. (Ancient Rome, usually in the plural) One of a group of ancient laws attributed to the Roman kings.
   • 27 BCE – 25 BCE, Titus Livius, Ab Urbe Condita 6.1.10:

     In primis foedera ac leges—erant autem eae duodecim tabulae et quaedam regiae leges—conquiri, quae comparerent, iusserunt.

     They first of all decreed that treaties and laws be collected—namely the twelve tables and certain royal laws—as far as they could be discovered.

2. (Late Latin, Medieval Latin) The irrevocable transfer of supreme power from the Roman people to the emperor, the basis of secular monarchy.
   • 1327 – 1400, Baldus de Ubaldis, Commentaria in Digesta ad 2.1.3:

     Prout est in principe diffinitur sic: merum imperium est absoluta potestas imperatori concessa per legem regiam.

     Just as, in the prince, it is defined thus: merum imperium is the absolute power conceded to the emperor through the lex regia.

Third-declension noun with a first-declension adjective.