One usage—the more common among linguists—refers to a variety of a language that is a characteristic of a particular group of the language's speakers. The term is applied most often to regional speech patterns, but a dialect may also be defined by other factors, such as social class. A dialect that is associated with a particular social class can be termed a sociolect, a dialect that is associated with a particular ethnic group can be termed as ethnolect, and a regional dialect may be termed a regiolect. According to this definition, any variety of a language constitutes "a dialect", including any standard varieties.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What does any variety of a linguist constitute?
unanswerable