Vulgar Latin, also known as Colloquial, Popular, Spoken or Vernacular Latin, is the range of non-formal registers of Latin spoken from the Late Roman Republic onward. Vulgar Latin as a term is both controversial and imprecise. Spoken Latin existed for a long time and in many places. Scholars have differed in opinion as to the extent of the differences, and whether Vulgar Latin was in some sense a different language. This was developed as a theory in the nineteenth century by Raynouard. At its extreme, the theory suggested that the written register formed an elite language distinct from common speech, but this is now rejected.

The current consensus is that the written and spoken languages formed a continuity much as they do in modern languages, with speech tending to evolve faster than the written language, and the written, formalised language exerting pressure back on speech. Vulgar Latin is used in different ways by different scholars, applying it to indicate spoken Latin of differing types, or from different social classes and time periods. Nevertheless, interest in the shifts in the spoken forms remains very important to understand the transition from Latin or Late Latin through to Proto-Romance and Romance languages. To make matters more complicated, evidence for spoken forms can be found only through examination of written Classical Latin, Late Latin, or early Romance, depending on the time period.

During the Classical period, Roman authors referred to the informal, everyday variety of their own language as sermo plebeius or sermo vulgaris, meaning "common speech". This could simply refer to unadorned speech without the use of rhetoric, or even plain speech. The modern usage of the term Vulgar Latin dates to the Renaissance, when Italian thinkers began to theorize that their own language originated in a sort of "corrupted" Latin that they assumed formed an entity distinct from the literary Classical variety, though opinions differed greatly on the nature of this "vulgar" dialect.

The early 19th-century French linguist François-Just-Marie Raynouard is often regarded as the father of modern Romance philology. Observing that the Romance languages have many features in common that are not found in Latin, at least not in "proper" or Classical Latin, he concluded that the former must have all had some common ancestor (which he believed most closely resembled Old Occitan) that replaced Latin some time before the year 1000. This he dubbed la langue romane or "the Romance language".

The first truly modern treatise on Romance linguistics and the first to apply the comparative method was Friedrich Christian Diez's seminal Grammar of the Romance Languages. Researchers such as Wilhelm Meyer-Lübke characterised Vulgar Latin as, to a great extent, a separate language that was more or less distinct from the written form. To Meyer-Lübke, the spoken Vulgar form was the genuine and continuous form, while Classical Latin was a kind of artificial idealised language imposed upon it; thus Romance languages were derived from the "real" Vulgar form, which had to be reconstructed from remaining evidence. Others that followed this approach divided Vulgar from Classical Latin by education or class. Other views of "Vulgar Latin" include defining it as uneducated speech, slang, or in effect, Proto-Romance.

The result is that the term "Vulgar Latin" is regarded by some modern philologists as essentially meaningless, but unfortunately very persistent:

the continued use of "Vulgar Latin" is not only no aid to thought, but is, on the contrary, a positive barrier to a clear understanding of Latin and Romance....

I wish it were possible to hope the term might fall out of use. Many scholars have stated that "Vulgar Latin" is a useless and dangerously misleading term ... To abandon it once and for all can only benefit scholarship.

Lloyd called to replace the use of "Vulgar Latin" with a series of more precise definitions, such as the spoken Latin of a particular time and place.