Wikipedia
or (, "leaning [of the hands]"), also (, " ordination"), (, "rabbinical ordination"), or (, "cantorial ordination") is derived from a Hebrew word which means to "rely on" or "to be authorized".
Prevailing generally refers to the ordination of a rabbi or cantor within post- talmudic Rabbinic Judaism, and within all modern Jewish religious movements from Reform to Orthodox. signifies the transmission of rabbinic authority to give advice or judgment in Jewish law. signifies the transmission of authoritative knowledge about Jewish musical and liturgical traditions. Although presently most functioning synagogue rabbis hold by some rabbinical institution or academy, this was until quite recently not always required, and in fact many Haredi rabbis may not be required to hold a "formal" even though they may occupy important rabbinical and leadership positions. Some cantorial institutions in the US currently grant to their students, while others use the term "investiture" to describe the conferral of cantorial authority onto their graduates.
Classical refers to a specific type of ordination that, according to traditional Jewish teaching, traces a line of authority back to Moshe ben Amram, The Men of the Great Assembly, and the Great Sanhedrin. The line of classical died out in the 4th or 5th century CE but it is widely held that a line of Torah conferment remains unbroken. Some believe evidence existed that classical was existent during the 12th century when from Lebanon and Syria were traveling to Israel in order to pass on Torah conferment to their students. Others, such as Rav Yisroel of Shklov (1770–1839), believed may not have been broken at all but that it continued outside of the land of Israel. Today many believe in the existence of an unbroken chain of rabbinical tradition dating back to the time of Moshe ben Amram ("Moses") and Yehoshua ben Nun ("Joshua") (See "The Unbroken Chain of Torah" below).
A third and distinct meaning of ("leaning") is the laying of hands upon an offering of a korban ("sacrifice") in the times of the Temple in Jerusalem, see Semikhah in sacrifices.