The term most commonly used by outsiders, including most American news organizations, is ultra-Orthodox Judaism. Hillel Halkin suggests the origins of the term may date to the 1950s, a period in which Haredi survivors of the Holocaust first began arriving in America. However, Isaac Leeser (1806–1868) was described in 1916 as "ultra-Orthodox".
Haredi is a Modern Hebrew adjective derived from the Biblical verb hared, which appears in the 66:5) and is translated as "[one who] trembles" at the word of God. The word connotes an awe-inspired fear and anxiety to perform the will of God; it is used to distinguish them from other Orthodox Jews (similar to the name used by Christian Quakers to describe their relationship to God).
The word Haredi is often used in the Jewish diaspora in place of the term ultra-Orthodox, which many view as inaccurate or offensive, it being seen as a derogatory term suggesting extremism; English-language alternatives that have been proposed include fervently Orthodox, strictly Orthodox, or traditional Orthodox. Others, however, dispute the characterization of the term as pejorative. Ari L. Goldman, a professor at Columbia University, notes that the term simply serves a practical purpose to distinguish a specific part of the Orthodox community, and is not meant as pejorative. Others, such as Samuel Heilman, criticized terms such as ultra-Orthodox and traditional Orthodox, arguing that they misidentify Haredi Jews as more authentically Orthodox than others, as opposed to adopting customs and practises that reflect their desire to separate from the outside world.
The community has sometimes been characterized as traditional Orthodox, in contradistinction to the Modern Orthodox, the other major branch of Orthodox Judaism, and not to be confused with the movement represented by the Union for Traditional Judaism, which originated in Conservative Judaism).
Haredi Jews also use other terms to refer to themselves. Common Yiddish words include Yidn ('Jews'), erlekhe Yidn ('virtuous Jews'), ben Torah ('son of the Torah'), frum ('pious'), and heimish ('home-like', i e.,'"our crowd').
In Israel, Haredi Jews are sometimes also called by the derogatory slang words dos (plural dosim), that mimics the traditional Ashkenazi Hebrew pronunciation of the Hebrew word datim, meaning 'religious', and more rarely, sh'chorim ('blacks'), a reference to the black clothes they typically wear; a related informal term used in English is black hat.