Ashkenazi Jews are a Jewish diaspora population who formed in the Holy Roman Empire around the end of the first millennium CE. They are the largest stream of Jewish people today, with unique customs, culture, language, and history. The term "Ashkenazi" refers to diaspora Jews who established communities along the Rhine in western Germany and northern France during the Middle Ages. Upon their arrival, they adapted traditions carried over from the Holy Land, Babylonia, and the western Mediterranean to their new European environment. The Ashkenazi religious rite developed in cities such as Mainz, Worms, and Troyes.

In a religious sense, an Ashkenazi Jew is any Jew whose family tradition and ritual follow Ashkenazi practice. In an ethnic sense, an Ashkenazi Jew is one whose ancestry can be traced to the Jews who settled in Central Europe. For roughly a thousand years, the Ashkenazim were a reproductively isolated population in Europe, despite living in many countries, with little inflow or outflow from migration, conversion, or intermarriage with other groups.

Today, about half of Jewish people around the world identify as Ashkenazi, meaning that they descend from Jews who lived in Central or Eastern Europe. Although strictly speaking, "Ashkenazim" refers to Jews of Germany, the term has come to refer more broadly to Jews from Central and Eastern Europe. The modern religious denominations developed in Ashkenazic countries, and therefore most North American synagogues use the Ashkenazic liturgy.

Ashkenazi Jewish ancestry is under the umbrella of "European ancestry," but it is clear from numerous studies that people of Ashkenazi ancestry are distinct from other European populations. While most people with Ashkenazi ancestry trace their DNA to Eastern and Central Europe, they are often more genetically like other Jewish populations, such as Sephardic Jews or Jewish groups with roots in Iran, Iraq, or Syria, than other Europeans.