One of the first lessons of Linguistics 101 is: “Everyone speaks a dialect.” We all come from somewhere, we all belong to some social class, we all have some gender, and all of these—plus many more things—affect our language. To a linguist, there’s no such thing as a “standard” dialect any more than to a surgeon there’s such a thing as a “standard” anatomy—everyone varies. To a linguist, there’s no real difference between a language and a dialect—as the redoubtable Ilse Lehiste put it, “A language is a dialect with an army and a navy.”
To a linguist, the term dialect generally refers to a form of language that is associated with a particular geographic area. Although the terminology varies from linguist to linguist, we have other words for varieties of language associated with particular social groups (sociolect), people who share very specific activities (jargon), levels of formality and specific social contexts (register), and so on. How you will speak at any given time is an interaction between the tendencies of your dialect, sociolect, the register, and so on.
Back to dialect: there is a cognate term in Spanish, but the denotation is quite different. In Spanish, dialecto typically refers to an indigenous language. As one of our surgeons, a native speaker of Spanish, put it the other day, referring to a patient in the recovery room: “even though I speak Spanish, I don’t understand him, because he speaks a dialect.” This was a patient who spoke one of the indigenous languages of Guatemala. Most of these languages belong to the Mayan language family. There are about 29 Mayan languages, of which 21 are spoken in Guatemala by the large indigenous population—about 70% of Guatemalans are native. (The rest are called Ladinos.) The Mayan languages are about as similar to or different from each other as English and German or Spanish and French. Patients who speak one of these languages and don’t also speak Spanish must bring someone who can interpret between their language and Spanish, and then one of the Surgicorps interpreters interprets between Spanish and English. I’ve seen four patients like that so far in the past three days.
This leads to the question: if the Spanish word dialecto means an indigenous language, how do you say “dialect” in Spanish? I don’t know the answer. The Spanish Wikipedia page for dialecto discusses it as a technical term in linguistics, with the same basic meaning as the English sense. (Note that as is the case in English, there is sometimes a large difference between the technical meaning of a term and the meaning of that term in the general language.) There is a Spanish term jerga that translates roughly as “jargon.” The Spanish Wikipedia page for jerga distinguishes it from “dialect” in that a jerga is associated with a social group or a profession, and is used either to obscure communication with out-group members (slang) or to enhance communication on technical subjects (jargon). The search continues.
Zipf's Law 