Kol Nidre (also known as Kol Nidrei and Kal Nidre and Col Nidre) (Aramaic: כָּל נִדְרֵי) is an Aramaic declaration recited in the synagogue before the beginning of the evening service on every Yom Kippur, the Day of Atonement. Though not a prayer, this dry legal formula and its ceremonial accompaniment have been charged with emotional undertones since the medieval period, creating a dramatic introduction to Yom Kippur on what is often dubbed "Kol Nidrei night". It is written in Aramaic, not Hebrew. Its name is taken from the opening words, meaning all vows.
Kol Nidrei has had an eventful history, both in itself and in its influence on the legal status of the Jews. Introduced into the liturgy despite the opposition of some rabbinic authorities, attacked in the course of time by some rabbis, and in the nineteenth century expunged from the prayer book by many communities of western Europe.
The term Kol Nidrei refers not only to the actual declaration, but is also popularly used as a name for the entire Yom Kippur evening service.
Read more about Kol Nidre: Form of The Chant, Origin, Language, Method of Recitation, Text, Explanation of Terms and Variants, Use By Non-Jews, Counterpoint, Reform in The Nineteenth Century, The Ashkenazi Melody, Similarities To Christian Plainsong, Popular Culture