Yiddish, historically Judeo-German or Jewish German, is a West Germanic language historically spoken by Ashkenazi Jews. It originated in 9th-century Central Europe and provided the nascent Ashkenazi community with a vernacular based on High German fused with many elements taken from Hebrew (notably Mishnaic) and to some extent Aramaic. Most varieties of Yiddish include elements of Slavic languages, and the vocabulary contains traces of Romance languages. Yiddish has traditionally been written using the Hebrew alphabet.
Before World War II, there were 11–13 million speakers. 85% of the approximately six million Jews who were murdered 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 among survivors after adapting to Modern Hebrew in Israel. However, the number of Yiddish speakers is increasing in Haredi communities. In 2014, YIVO stated that "most people who speak Yiddish in their daily lives are Hasidim and other Haredim", whose population was estimated at the time to be between 500,000 and 1 million. A 2021 estimate from Rutgers University was that there were 250,000 American speakers, 250,000 Israeli speakers, and 100,000 in the rest of the world (for a total of 600,000).