Cathar

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Catharism

Recorded since the 16th century; from the masculine plural of Medieval Latin Catharī (“Pure (ones)”), from the masculine plural of Byzantine Greek καθαροί (katharoí, “Pure (ones)”), from the masculine singular of Byzantine Greek καθαρός (katharós, “Pure (one)”), from Ancient Greek καθαρός (katharós, “pure”).

Cathar (plural Cathari or Cathars)

A member of certain so self-styled Novatian and other medieval Christian sects embracing a form of dualism and extraordinary practices purportedly adhering to Mary Magdalene's teachings, persecuted by Roman Catholics as heretics.
The Albigenses, famous Cathars in and around Albi (southern France), were eradicated in a bloody ‘crusade’.

Most often used in the plural.

member of one or more medieval Christian sects