The earliest surviving references date from the 12th century and call the language לשון־אַשכּנז (loshn-ashknaz, "language of Ashkenaz") or טײַטש (taytsh), a variant of tiutsch, the contemporary name for Middle High German. Colloquially, the language is sometimes called מאַמע־לשון (mame-loshn, lit. "mother tongue"), distinguishing it from לשון־קודש (loshn koydesh, "holy tongue"), meaning Hebrew and Aramaic. The term "Yiddish", short for Yidish Taitsh ("Jewish German"), did not become the most frequently used designation in the literature until the 18th century. In the late 19th and into the 20th century the language was more commonly called "Jewish", especially in non-Jewish contexts,[clarification needed] but "Yiddish" is again the more common designation today.
Modern Yiddish has two major forms. Eastern Yiddish is far more common today. It includes Southeastern (Ukrainian–Romanian), Mideastern (Polish–Galician–Eastern Hungarian), and Northeastern (Lithuanian–Belarusian) dialects. Eastern Yiddish differs from Western both by its far greater size and by the extensive inclusion of words of Slavic origin. Western Yiddish is divided into Southwestern (Swiss–Alsatian–Southern German), Midwestern (Central German), and Northwestern (Netherlandic–Northern German) dialects. Yiddish is used in a number of Haredi Jewish communities worldwide; it is the first language of the home, school, and in many social settings among many Haredi Jews, and is used in most Hasidic and some Lithuanianyeshivas.
The term "Yiddish" is also used in the adjectival sense, synonymously with "Jewish", to designate attributes of Yiddishkeit ("Ashkenazi culture"; for example, Yiddish cooking and "Yiddish music" - klezmer).
Prior to the Holocaust, there were 11–13 million speakers of Yiddish among 17 million Jews worldwide. 85% of the approximately 6 million Jews who died in the Holocaust were Yiddish speakers, leading to a massive decline in the use of the language. Assimilation following World War II and aliyah, immigration to Israel, further decreased the use of Yiddish both among survivors and among Yiddish-speakers from other countries (such as in the Americas). However, the number of speakers is increasing in Hasidic communities.
The established view is that, as with other Jewish languages, Jews speaking distinct languages learned new co-territorial vernaculars, which they then Judaized. In the case of Yiddish, this scenario sees it as emerging when speakers of Zarphatic (Judaeo-French) and other Judaeo-Romance languages began to acquire varieties of Middle High German, and from these groups the Ashkenazi community took shape. Exactly what German base lies behind the earliest form of Yiddish is disputed.
In Max Weinreich's model, Jewish speakers of Old French or Old Italian who were literate in either liturgical Hebrew or Aramaic, or both, migrated through Southern Europe to settle in the Rhine Valley in an area known as Lotharingia (later known in Yiddish as Loter) extending over parts of Germany and France; There, they encountered and were influenced by Jewish speakers of High German languages and several other German dialects. Both Weinreich and Solomon Birnbaum developed this model further in the mid-1950s. In Weinreich's view, this Old Yiddish substrate later bifurcated into two distinct versions of the language, Western and Eastern Yiddish. They retained the Semitic vocabulary and constructions needed for religious purposes and created a Judaeo-German form of speech, sometimes not accepted as a fully autonomous language.
Later linguistic research has finessed the Weinreich model or provided alternative approaches to the language's origins, with points of contention being the characterization of its Germanic base, the source of its Hebrew/Aramaic adstrata, and the means and location of this fusion. Some theorists argue that the fusion occurred with a Bavarian dialect base. The two main candidates for the germinal matrix of Yiddish, the Rhineland and Bavaria, are not necessarily incompatible. There may have been parallel developments in the two regions, seeding the Western and Eastern dialects of Modern Yiddish. Dovid Katz proposes that Yiddish emerged from contact between speakers of High German and Aramaic-speaking Jews from the Middle East. The lines of development proposed by the different theories do not necessarily rule out the others (at least not entirely); an article in The Forward argues that "in the end, a new 'standard theory' of Yiddish’s origins will probably be based on the work of Weinreich and his challengers alike."
Paul Wexler proposed a model in 1991 that took Yiddish, by which he means primarily eastern Yiddish, not to be genetically grounded in a Germanic language at all, but rather as "Judaeo-Sorbian" (a proposed West Slavic language) that had been relexified by High German. In more recent work, Wexler has argued that Eastern Yiddish is unrelated genetically to Western Yiddish. Wexler's model has been met with little academic support, and strong critical challenges, especially among historical linguists.