Latin has been divided into historical phases, each of which is distinguished by minor differences in vocabulary, usage, spelling, morphology and syntax. In addition to the historical phases, Ecclesiastical Latin refers to the styles used by the writers of the Roman Catholic Church in all historical phases from Late Latin on.
Old, early or archaic Latin
The earliest known is Old Latin, a phase of the early and middle Roman republic attested in inscriptions and the earliest surviving Latin works of literature.
Old Latin was followed in the late republic and empire by Classical Latin, a conscious creation of the orators, poets, historians and other literate men, who wrote the great works of classical literature, and was taught in the schools of grammar and rhetoric. The concepts of today's instructional grammars originated in these schools, which served as a sort of informal language academy to maintain and perpetuate the classical language.
After the fall of the Western Roman Empire, literary Latin survived as the lingua franca of educated classes in the West. The population of the Eastern Roman Empire used a form of Greek that descended to modern Greek, even though the administration assumed names and titles that had come from Latin. The eastern empire survived until dissolved by the developing Turkish nation.
Philological analysis of Old Latin works, such as the plays of Plautus, which contain dialogue purporting to be the speech of the common people, indicates that contemporaneous with the literary and official language was a spoken language, which has from ancient times been called Vulgar Latin (sermo vulgi in Cicero), the language of the vulgus or "common people." Since the vulgus spoke — but did not write their language — it can only be known through words and phrases cited by classical authors or in inscriptions.
As vulgar Latin was not under the control or encouragement of the schools of rhetoric, there is no reason to expect any uniformity of speech either diachronically or geographically. Just the opposite must have been true: European populations learning Latin developed their own dialects of the language. This is the situation that prevailed when the Migration Period, ca. 300-700 AD, brought an end to the unity and peace of the Roman world and removed the stabilizing influence of its institutions on the language. A post-classical phase of Latin appeared, Late Latin, in which the spoken forms reappeared, and which is regionalized. Starting about the 5th or 6th centuries, Late Latin contains minor features that are germinal to the development of the Romance languages.
One of the tests as to whether a given Latin feature or usage was in the spoken languge is to compare its reflex in a Romance language with the equivalent structure in classical Latin. If it appeared in the Romance language but was not preferred in classical Latin, then it passes the test as being vulgar Latin. For example, grammatical case in nouns is present in classical Latin but not in the Romance languages, excluding Romanian. One might conclude that case endings in regions other than Romania were already wholly or partly missing in the spoken language even while being insisted upon in the written. (Even in Romanian there are as many case endings for nouns as there are for pronouns in the other languages; cf. Romanian endings i, lor with the Italian pronouns gli, loro). Much of the vocabulary also that went into the Romance languages came from Vulgar Latin rather than classical. The following examples follow the formula, classical Latin word/vulgar Latin word/French word: ignis/focus/feu, equus/caballus/cheval, loquor/parabolare/parler, pulcher/bellus/bel (or belle)In each case French does not use the classical Latin word. The words actually used: focus, caballus, etc., must have been in the Vulgar Latin vocabulary.
The expansion of the Roman Empire had spread Latin throughout Europe. Vulgar Latin began to diverge into various dialects and many of these into distinct Romance languages by the 9th century at very latest, when the earliest known writings appeared. The languages must already have been in place. These were, for many centuries, only oral languages, Latin still being used for writing. For example, Latin was still the official language of Portugal until 1296, when Portuguese replaced it. Portuguese had already developed and was in use under the umbrella of the vulgar language.
The term Mediaeval Latin refers to the written Latin in use during that portion of the post-classical period when no corresponding Latin vernacular existed. The spoken language had developed into the various incipient Romance Languages; however, in the educated and official world Latin continued without its spoken base. Moreover, this Latin spread into lands that had never spoken Latin, such as the Germanic and Slavic nations. It became useful as a means of international communication between the member states of the Holy Roman Empire and its allies.
Cut loose from its corrective spoken base and severed from the vanished institutions of the Roman empire that had supported its uniformity, mediaeval Latin lost the precise knowledge of correctness; for example, suus ("his/her own") and eius ("his/her") are used interchangeably, an error that would have been swiftly corrected in the schools of classical Rome. In classical Latin sum and eram are used as auxiliary verbs in the perfect and pluperfect passive, which are compound tenses. Mediaeval Latin might use fui and fueram instead. Furthermore the meanings of many words have changed and new vocabulary has been introduced from the vernacular.
While these minor changes are not enough to impair comprehension of the language, they introduce a certain flexibility not in it previously. The style of each individual author is characterized by his own uses of classically incorrect Latin to such a degree that he can be identified just by reading his Latin. In that sense mediaeval Latin is a collection of individual Latins united loosely by the main structures of the language. Some are more classical, others less so.As the majority of these writers were influential members of the Christian church: bishops, monks, philosophers, etc., the term Ecclesiastical Latin does not accurately apply; the majority were ecclesiastical by occupation but there was no uniform language of the church; that was a product of the Renaissance. Late Latin is sometimes classified as mediaeval, sometimes not. Certainly many of the individual Latins were influenced by the vernaculars of their authors.
The Renaissance briefly reinforced the position of Latin as a spoken language, through its adoption by the Renaissance Humanists. Often led by members of the clergy, they were shocked by the accelerated dismantling of the vestiges of the classical world and the rapid loss of its literature. They strove to preserve what they could. It was they who introduced the practice of producing revised editions of the literary works that remained by comparing surviving manuscripts, and they who attempted to restore Latin to what it had been. They corrected mediaeval Latin out of existence no later than the 15th century and replaced it with more formally correct versions supported by the scholars of the rising universities, who attempted, through scholarship, to discover what the classical language had been.