A Rebbe (רבי) or Admor (אדמו״ר) is the spiritual leader in the Hasidic movement, and the personalities of its dynasties. The titles of Rebbe and Admor, which used to be a general honor title even before the beginning of the movement, became, over time, almost exclusively identified with its Tzaddikim.
Today, rebbe is used in the following ways:
Rabbi, a teacher of Torah – Yeshiva students or cheder (elementary school) students, when talking to their teacher, would address him with the honorific Rebbe, as the Yiddish-German equivalent to the Hebrew word rabbi ( ˈʁabi).
Personal mentor and teacher—A person's main Rosh Yeshiva, Yeshiva teacher, or mentor, who teaches him or her Talmud and Torah and gives religious guidance, is referred to as rebbe (ˈrɛbə), also as an equivalent to the term "rabbi".
Spiritual leader—The spiritual head of a Hasidic movement is called a rebbe (ˈrɛbə). His followers would address him as "The Rebbe" or refer to him when speaking to others as "the Rebbe" or "my Rebbe". He is referred to by others as the Rebbe of a particular Hasidic dynasty. In Hebrew, a Hasidic rebbe is often referred to as an AdMoR, which is an abbreviation for Adoneinu, Moreinu, veRabbenu ("Our Master, our Teacher, and our Rabbi"). In writing, this title is placed before the name of the Hasidut, as in "Admor of Belz"; while the title Rebbe comes after the name of the Hasidut when used as an adjective, as in "Lubavitcher Rebbe", "Amshinever Rebbe", and every rebbe of every Hasidic dynasty. In the Litvishe world, when not referring to a hasidic rebbe (ˈrɛbə), the word can be pronounced "rebbee" (ˈrɛbi). Sephardic Jews can pronounce it as "Ribbi" (ˈriːbi). The Lubavitcher hasidim have a tradition that the Hebrew letters that make up the word rebbe (ˈrɛbi) are also an acronym for "Rosh Bnei Yisroel", meaning "a spiritual head of the Children of Israel".
An ordinary communal rabbi, or rebbe in Yiddish, is sometimes distinct from a rav (ˈræv, also pronounced rov ˈrɒv by Jews of Eastern European or Russian origin), who is a more authoritative halakhic decider.