Being a written language, Late Latin is not identical with Vulgar. The latter served as Proto-Romance, a reconstructed ancestor of the Romance languages. Although Late Latin reflects an upsurge of the use of Vulgar Latin vocabulary and constructs, it remains largely classical in its overall features, depending on the author. Some are more literary and classical, but some are more inclined to the vernacular. Also, Late Latin is not identical to Christian or to patristic Latin, the theological writings of the early Christian fathers. While Christian writings are considered a subset of Late Latin, pagans wrote much Late Latin, especially in the early part of the period.
Late Latin formed while mercenaries from non-Latin-speaking peoples on the borders of the empire were being subsumed and assimilated in large numbers, and the rise of Christianity was introducing a heightened divisiveness in Roman society, creating a greater need for a standard means of communicating between different socioeconomic registers and widely separated regions of the sprawling empire. A new and more universal speech evolved from the main elements: Classical Latin, Christian Latin, which featured sermo humilis, "ordinary speech" in which the people were to be addressed, and all the various dialects of Vulgar Latin. The linguist Antoine Meillet said, "without the exterior appearance of the language being much modified, Latin became in the course of the imperial epoch a new language”, and “serving as some sort of lingua franca to a large empire, Latin tended to become simpler, to keep above all what it had of the ordinary ...”.
Neither Late Latin nor Late Antiquity are modern terms or concepts; their origin remains obscure. Neither are they ancient; a notice in Harper's New Monthly Magazine of the publication of Andrews' Freund's Lexicon of the Latin Language in 1850 mentions that the dictionary divides Latin into ante-classic, quite classic, Ciceronian, Augustan, post-Augustan and post-classic or late Latin, which indicates the term already was in professional use by English classicists in the early 19th century. Instances of English vernacular use of the term may also be found from the 18th century. The term Late Antiquity meaning post-classical and pre-medieval had currency in English well before then.
Wilhelm Sigismund Teuffel's first edition (1870) of History of Roman Literature defined an early period, the Golden Age, the Silver Age and then goes on to define other ages first by dynasty and then by century (see under Classical Latin). In subsequent editions he subsumed all periods under three headings: the First Period (Old Latin), the Second Period (the Golden Age) and the Third Period, "the Imperial Age", subdivided into the Silver Age, the 2nd century, and Centuries 3–6 together, which was a recognition of Late Latin, as he sometimes refers to the writings of those times as "late." Imperial Latin went on into English literature; Fowler's History of Roman Literature mentions it in 1903.
There are, however, insoluble problems with the beginning and end of Imperial Latin. Politically the excluded Augustan Period is the paradigm of imperiality, and yet the style cannot be bundled with either the Silver Age or with Late Latin. Moreover, in 6th century Italy, the Roman Empire no longer existed; the rule of Gothic kings prevailed. Subsequently the term Imperial Latin was dropped by historians of Latin literature, although it may be seen in marginal works. The Silver Age was extended a century and the final four centuries represent Late Latin.