From The Art and Popular Culture Encyclopedia
Patronus (plural patroni) was part of the social customs of Ancient Rome, a social term that referred to the senior party in one of several social relationships.
It could refer to the protector, sponsor, or benefactor of a cliens from the lower classes or from outside Rome in a relationship called clientela. This social institution was ancient, extending into Rome's earliest society; the Romans believed it was invented by Romulus. In the earliest periods, patricians were patrons of plebian citizens.
By the time of Cicero, the patroni could also refer to the more professional relationship between an advocate and his legal client and was a rough synonym for the term advocatus.
In Cicero's time, a prominent Roman senator might be the patronus of an entire city or foreign country. Sometimes, the Roman Senate would settle disputes between foreign nations within its "sphere of influence" by referring the matter to be resolved between the patroni of the respective disputing client nations, and abide by the results.
A Patronus had certain obligations towards their cliens, including that of a being a legal advisor and protector.
Furthermore, various professional and other corporations awarded statutory titles such as patronus or pater patratus (some used more than one) to external protectors, such as Magistrates or members of the imperial family, sometimes merely as a flattery, rather like modern charities enjoying the 'high protection' of one or more princes, statesmen etcetera with or without any actual involvement.