Named after Zayd ibn Ali, son of the fourth Imam, Ali ibn Husayn, whom they consider to be have inherited the Imamate (rather than his brother Muhammad al Baqir, the lineage accepted by other Shi'a).
They are the dominant religious group in modern Yemen. Their beliefs and practices are so close to Sunni Islam that they are sometimes referred to as a fifth "madhhab", or school of Sunni jurisprudence.