Split from here Well, Yiddish is a dialect of High German just as Bavarian would be considered a dialect of High German. One caveat though: There is no clear definition of what constitutes the difference between a dialect and a language. So while Madarin and the language spoken in Guangzhou (south China) are considered topolectal variations of the same language, English and German are considered different languages. There is no clear linguistic corollary for this distinction. In the case of Yiddish, it is structurally extremely close to modern high German inspite of all the lexical imports from Slavic languages. It's a bit like Romanian which is generally considered a romance language although its vocab has many Slavic imports. Likewise Spanish is classified as a romance language even though many lexical items have Arabic roots. So Yiddish should definitely not be called a Slavic language, because some Slavic imports don't necessarily have much of an impact on structure. For all I know about Yiddish, and I have to rely on the experts, it is extremely close to High German as opposed to Low German. The reference to High German means that contrary to most other West-Germanic languages like English for example, the phonology and phonetics of Yiddish bear all the hallmarks of the High German sound shift. It's morphology is clearly derived from High German. These are the principal areas where a linguist would look to determine linguistic family resemblance. So Yiddish is much closer to High German than for example Plattdeutsch. We can classify it as a language, but then of course we would have to call Plattdeutsch a language as well. Some linguists do the latter but still wouldn't call Yiddish a language.