What was Vulgar Latin?

1. Variation within Latin (socially, geographically, and chronologically) that differs from the Classical literary standard in an age when most people were illiterate and the primary method of language transmission between people was oral. This typically excludes the language of the more educated upper classes, which, although it does include variation, comes closest to the standard.
2. The spoken Latin of the Roman Empire. Classical Latin represents the literary register of Latin, based on the model of ancient literary Greek. It represented a selection from a variety of spoken forms. The Latin brought by Roman soldiers to Gaul, Iberia or Dacia wasn't identical to the Latin of Cicero, and differed from it in vocabulary, and later in syntax and grammar as well. By this definition, Vulgar Latin was a spoken language and Classical Latin was used for writing, with the later style of literary Latin being slightly different, showing greater influence from the vulgar dialects, compared to the earlier "classical" standards.
3. The hypothetical ancestor of the Romance languages ("Proto-Romance"), which can't be directly known, except from a few graffiti inscriptions. Proto-Romance is a hypothetical vernacular derived from Latin that had undergone important and varying sound shifts and other changes which can be reconstructed from the changes evident in its descendants, the Romance languages.
4. "Vulgar Latin" is sometimes used to describe the grammatical changes found in some Late Latin texts, such as the 4th-century Itinerarium Egeriae, Egeria's account of her journey to Palestine and Mt. Sinai; or the works of St Gregory of Tours. Since written documentation of Vulgar Latin forms is scarce, these works are invaluable to philologists, mainly because of the occasional presence of variations or errors in spelling, providing some evidence of spoken usage during the period in which they were written.
      Some literary works with a lower register of Latin also provide a glimpse into the world of early Vulgar Latin. The works of Plautus and Terence, being comedies with many characters who were slaves, preserve some early basilectal Latin features, as does the recorded speech of freedmen in the Cena Trimalchionis by Petronius Arbiter.
      For many centuries after the fall of the Roman Empire in the West, Vulgar Latin continued to coexist with a written form of Late Latin, nowadays referred to as Medieval Latin; for when speakers of Romance vernaculars set out to write with correct grammar and spelling, they attempted to emulate the norms of Classical Latin. This scholarly Latin, "frozen" by Justinian's codifications of Roman law on the one hand, and by the Catholic Church on the other, was eventually unified by the medieval copyists; it continued to exist as a Dachsprache in the Middle Ages, and a lingua franca well beyond them.
      Vulgar Latin developed differently in the various provinces of the Roman Empire, gradually giving rise to such modern languages as French, Catalan, Italian, Spanish, Portuguese, and Romanian. Although the official language in these areas was Latin, Vulgar Latin was popularly spoken until the new localized forms diverged sufficiently from Latin, thus emerging as separate languages. However, despite the widening gulf between the spoken and written Latin, throughout the imperial era and until the 8th century CE, it wasn't significant enough as to make them mutually unintelligible. József Herman states:
   

   It seems certain that in the sixth century, and quite likely into the early parts of the seventh century, people in the main Romanized areas could still largely understand the biblical and liturgical texts and the commentaries (of greater or lesser simplicity) that formed part of the rites and of religious practice, and that even later, throughout the seventh century, saints' lives written in Latin could be read aloud to the congregations with an expectation that they'd be understood. We can also deduce however, that in Gaul, from the central part of the eighth century onwards, many people, including several of the clerics, were not able to understand even the most straightforward religious texts

   
   Indeed, at the third Council of Tours in 813, priests were ordered to preach in the vernacular language — either in the rustica lingua romanica (Vulgar Latin), or in the Germanic vernaculars — since the common people could no longer understand formal Latin. Within a generation, the Oaths of Strasbourg (842), a treaty between Charlemagne's grandsons Charles the Bald and Louis the German, was proffered and recorded in a language that was already distinguished from Latin. Consider the excerpt below:
   

   Pro Deo amur et pro christian poblo et nostro commun salvament, d'ist di in avant, in quant Deus savir et podir me dunat, si salvarai eo cist meon fradre Karlo et in ajudha et in cadhuna cosa, si cum om per dreit son fradra salvar dift, in o quid il me altresi fazet, et ab Ludher nul plaid numquam prindrai, qui, meon vol, cist meon fradre Karle in damno sit. For the love of God and for Christendom and our common salvation, from this day onwards, as God will give me the wisdom and power, I'll protect this brother of mine Charles, with aid or anything else, as one ought to protect one's brother, so that he may do the same for me, and I'll never knowingly make any covenant with Lothair that would harm this brother of mine Charles.

   
   From this point on, the Latin vernaculars began to be treated as separate languages in practice, developing local norms and orthographies of their own, and "Vulgar Latin" ceases to be a useful term.

   Vocabulary

   Classical Latin	Vulgar Latin	English
   bellum	*guerra	war
   cogitare	pensare	think
   edere	*manducari	eat
   emere	comparare	buy
   equus	caballus	horse
   feles	catta	cat
   hortus	*gardinus	garden
   ignis	focus	fire
   ludere	iocari	play
   omnis	totus	all
   os	bucca	mouth
   pulcher	bellus	beautiful
   urbs	civitas	city
   verbum	parabola	word
   vesper	sera	evening

   Certain words from Classical Latin were dropped from the vocabulary. Classical equus, "horse", was consistently replaced by caballus "nag" (but note Romanian iapă, Sardinian èbba, Spanish yegua, Catalan euga and Portuguese égua all meaning "mare" and deriving from Classical equa).
      A sample of words that are exclusively Classical, and those that were productive in Romance, is to be found in the table to the right.
      The vocabulary changes affected even the basic grammatical particles of Latin; there are many that vanish without a trace in Romance, such as an, at, autem, donec, enim, ergo, etiam, haud, igitur, ita, nam, postquam, quidem, quin, quod, quoque, sed, utrum and vel.
      Verbs with prefixed prepositions frequently displaced simple forms. The number of words formed by such suffixes as -bilis, -arius, -itare and -icare grew apace. These changes occurred frequently to avoid irregular forms or to regularise genders.
      On the other hand, since Vulgar Latin and Latin proper were for much of their history different registers of the same language, rather than different languages, some Romance languages preserve Latin words that were lost in most others. For example, Italian ogni ("each/every") preserves Latin omnes. Other languages use cognates of totus for the same meaning; for example tutto in Italian, tudo/todo in Portuguese, todo in Spanish, tot in Catalan, tout in French and tot in Romanian.
      Sometimes, a classical Latin word was kept alongside a Vulgar Latin word. In Vulgar Latin, classical caput, "head", yielded to testa (originally "pot") in some forms of western Romance, including French and Italian. But Italian, French and Catalan kept the Latin word under the form capo, chef, and cap which retained many metaphorical meanings of "head", including "boss". The Latin word with the original meaning is preserved in Romanian cap, together with țeastă, both meaning 'head' in the anatomical sense. Southern Italian dialects likewise preserve capo as the normal word for "head". Spanish and Portuguese have cabeza/cabeça, derived from *capetia, a modified form of caput, while in Portuguese testa was retained as the word for "forehead".
      Frequently, words borrowed directly from literary Latin at some later date, rather than evolved within Vulgar Latin, are found side by side with the evolved form. The (lack of) expected phonetic developments is a clue that one word has been borrowed. For example, Vulgar Latin fungus, "fungus, mushroom", which became Italian fungo, Catalan fong, and Portuguese fungo, became hongo in Spanish, showing the f > h shift that was common in early Spanish (cf. filius > Spanish hijo, "son", facere > Spanish hacer, "to do"). But Spanish also had fungo, which by its lack of the expected sound shift shows that it was borrowed directly from Latin. written on the margins of a copy of the Vulgate Bible, suggesting that the 4th-century Vulgate words were no longer readily understood in the 8th century, when the glosses were likely written. These glosses demonstrate typical vocabulary changes in Gallo-Romance.
      The Reichenau Glosses show vocabulary replacement: