Final answer:
The regional variation of language featuring unique vocabulary, spelling, and pronunciation is known as a dialect. It can be associated with specific geographical regions or social groups, and over time, dialects may even evolve into separate languages.
Step-by-step explanation:
A regional variation of a language that includes distinctive vocabulary, spelling, and pronunciation of words is referred to as a dialect. A dialect is not only limited by geographical regions but can also be defined by social or ethnic groups. For example, the dialects found in the United States vary based on multiple factors, including location, culture, ethnicity, and socioeconomic status. Each dialect can have its own unique grammar, vocabulary, syntax, and pronunciation, much like how the dialect in Boston may non-rhotically pronounce 'car' as 'cah'.
In addition to geographical dialects, there are also social dialects, which are differences in speech associated with a social group or socioeconomic level. Over time, dialects can develop distinct linguistic features to the extent that they may be considered separate languages, although the distinction between a fully-developed dialect and a language can often be political. For instance, 'Standard American English,' used by newscasters, is considered a dialect just as much as 'African American Vernacular English' or 'Appalachian English' is.