The title vir illustris ('illustrious man') is used as a formal indication of standing in late antiquity to describe the highest ranks within the senates of Rome and Constantinople. All senators had the title vir clarissimus ('very famous man'); but from the mid fourth century onwards, vir illustris and vir spectabilis ('admirable man', a lower rank than illustris) were used to distinguish holders of high office.
The custom of Roman senators of late antiquity appending the title of vir clarissimus to their names developed gradually over the first two centuries. During the fourth century, the senatorial order greatly increased in number, so that the title became more common and new titles were devised to distinguish senators of a higher dignity, namely vir spectabilis and vir illustris. The first instance of vir illustris occurred in AD 354 with its use by the Praefectus praetorio. For some decades it was used inconsistently, but then more regularly, perhaps in connection with a formal codification of honours by Emperor Valentinian I in AD 372.
The offices that had a right to the title varied with time. The Notitia Dignitatum of the early AD fifth century attached it to the offices of the:
The Illustres soon were regarded as the active membership of the Senate; and by the middle of the AD fifth century, Spectabiles and Clarissimi were no longer expected to participate in the Senate. By the reign of Emperor Justinian I, all senators were considered Illustres. At the same time the title of "illustris" had been similarly devalued below that of "clarissimus" in the AD fourth century, and high officials were indicated with the titles of "vir gloriosus" or "gloriosissimus" and "vir magnificus".
In ancient inscriptions and manuscripts, the spelling "inlustris" is more frequent. Because the illustres were a subset of the clarissimi, the title is often written as "vir clarissimus et illustris", especially in official documents. The shorter title was commonly abbreviated "v. i." (plural "vv. ii."), "v. inl.", or "vir inl." and the longer title as "v. c. et inl."
In Merovingian and Carolingian times, the spellings vir inluster and viri inlustres were common.